 Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Hi guys, I'm using a lot of plaster forms on my layout and I am being thwarted by inconsistencies with painting. The problem is a lot of my casts are not soaking in the water paint as they should but seem to be repelling it as though the plaster is sealed (when its not) I have tried painting soon after casting or many weeks later, though I have not been methodical about that. I am using the same brand plaster, fresh and cast fresh. I use a flow agent on all my casts, provided by the mould supplier (spoerle). Can anyone explain what is causing this problem? Is it my plaster mix, or flow agent, or the timeframe after which I start painting? or something else? Examples below, two pieces that would not soak any paint:  And here we have another two examples. The top one was perfect in absorbing the paint and was a test piece. The one below absorbed paint but certain areas didn't, so still unusable:  My original thought was since I am in Brisbane that the high humidity is causing the casts to absorb moisture from the air and seal themselves after a certain time of exposure. However I then tried painting a really old cast and it was perfect, so then I thought I was not waiting long enough for the plaster to cure. That was also inconclusive as other old casts I have painting would not absorb paint at all! |
modelling era IIIa (1951-1955) Germany |
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Joined: 15/08/2012(UTC) Posts: 234 Location: Perth, Western Australia.
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Hi Applor, Im wondering if like us in Perth (Australia) you are forced to pay for drinking water that has contaminants in it? In our situation it is loaded with calcium. Perhaps a test piece using distilled or demineralised water might be a start?
Regards...........Chook.
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 1 user liked this useful post by Chook
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Originally Posted by: Chook  Hi Applor, Im wondering if like us in Perth (Australia) you are forced to pay for drinking water that has contaminants in it? In our situation it is loaded with calcium. Perhaps a test piece using distilled or demineralised water might be a start?
Regards...........Chook. Hi Chook thanks for the reply - In regards to your suggestion a number of my casts are able to be painted without problem. So I think that rules out water and plaster, more likely to be cure time or casting related. |
modelling era IIIa (1951-1955) Germany |
1 user liked this useful post by applor
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Joined: 20/07/2007(UTC) Posts: 809 Location: Leopold, Victoria
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Some plaster mixes have a bit of PVA glue mixed in to re- enforce it. |
Legless
Era's 1 to 111,C track,k track |
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Joined: 04/12/2013(UTC) Posts: 2,261 Location: Hobart, Australia
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Originally Posted by: applor Hi guys,
I'm using a lot of plaster forms on my layout and I am being thwarted by inconsistencies with painting.
The problem is a lot of my casts are not soaking in the water paint as they should but seem to be repelling it as though the plaster is sealed (when its not)
I have tried painting soon after casting or many weeks later, though I have not been methodical about that. I am using the same brand plaster, fresh and cast fresh.
I use a flow agent on all my casts, provided by the mould supplier (spoerle).
Can anyone explain what is causing this problem? Is it my plaster mix, or flow agent, or the timeframe after which I start painting? or something else?
My original thought was since I am in Brisbane that the high humidity is causing the casts to absorb moisture from the air and seal themselves after a certain time of exposure.
However I then tried painting a really old cast and it was perfect, so then I thought I was not waiting long enough for the plaster to cure.
That was also inconclusive as other old casts I have painting would not absorb paint at all! Hello Eric, I used plaster cast for rock formations but also a number of walls and I recall the same problem. In both areas my casting used some very dilute detergent to "wet" the moulds. It was not until I read you post that I now suspect the reason. My use of Plaster of Paris on my layout was mostly for rock formations à la Woodland Scenics :     However some were for pillars, walls and tunnel portals I made, constructed using casts of stoneface from an English model building construction system (can't recall name and materials stored deep in archive):     In retrospect I did notice the same effect : specifically the walls would not take waterbased paint but I found I could readily paint them with Humbrol matt enamels. In contrast I had no problems getting the Woodland system to "take" waterbased stains. Whilst this may be inherent in the Woodland stains, in the Woodlands system one "seals" the surface before each coat with a spray of what is essentially dilute white glue. So, my advice : try a seal of the Woodland Scenics spray (or equivalent) and failing this use Humbrol matt enamel. Regards, PJ
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1 user liked this useful post by PJMärklin
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 Joined: 15/09/2014(UTC) Posts: 682 Location: London
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Have you tried adding some paint when you mix the plaster? It will at least help to hide any chips and it may also help any final paint covering to bond to the casting.
Carim
p.s. I think the casting system that PJ was referring to was "Linka"
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1 user liked this useful post by Carim
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 Joined: 04/04/2013(UTC) Posts: 1,291 Location: Port Moody, BC
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I mainly use Hydrocal for my rock castings and Woodland Scenics pigment dilutions for the colouring. Hydrocal absorbs the pigment mixtures quite well, whereas I have found that plaster of Paris poured into the same moulds hardly accepts the pigment mixture and I now only use it for a base coat covered by Hydrocal.
Regarding Woodland Scenics and their sealing mixture, one of their tutorial videos shows applying the various pigments, sealing the pigmented product with their sealer, and then adding a final pigment coating and final sealer. I tried that and once the first sealer coat was sprayed on I could not add any further pigment colouring. I contacted Woodland Scenics about this and was advised that once the sealer was applied no further pigment mixture would be absorbed and that they would correct their tutorial. However, the last time I checked this tutorial had not been corrected.
So as someone else has pointed out, once a sealer or PVA mixture is involved, pigment mixtures will just not work. At that point you need to use paints either with a brush or air brush.
Peter
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1 user liked this useful post by PMPeter
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Hi all,
Yes I could paint with acrylic or enamel paints over the top of my 'bad' casts (since they behave as though sealed) but I am wanting to use water colours on the plaster to achieve the realistic soft blend of colours.
More so, that is just working around the problem rather than fixing the cause and I wouldn't know before I started if the plaster was going to accept water colour paint or not.
I had done some tets with painting weeks ago with some plaster rock casting using the WS method (which were absorbing water colour fine) and yes applying glue will seal the coat.
Sealing the coat like in the WS videos should only be done after all base colours have been applied so that after sealing, the black doesn't soak into the plaster but runs into the cracks and dries to provide depth.
So yes I also found the tutorial was not quite correct as there shouldn't be a sealing layer done at all prior to moving to the black wash stage.
I have also enquired on Stummi forums for as much exposure and experience as possible.
So far they are certain the wetting agent to be the cause and said not to use any.
Curious that Spoerle includes the flow agent and instructions with the moulds and this problem is not mentioned at all.
I also thought I had tried some casts without flow agent and still had the same problem but I may be mistaken.
Some also advised to paint the casts with a primer before painting. I am not sure how this would work with water colour paints though but I could try it on the casts I have done.
So next step I am going to do some casts without any flow agent and try painting a week later to see how they go.
I can either try a primer on my existing casts or discard them.
Luckily plaster is cheap and I can do as many casts as I want.
The downside is I have spent the last few months doing dozens of casts which may have been wasted time (which is very depressing and demoralising given what little time I have) |
modelling era IIIa (1951-1955) Germany |
1 user liked this useful post by applor
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Joined: 04/12/2013(UTC) Posts: 2,261 Location: Hobart, Australia
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Originally Posted by: Carim Have you tried adding some paint when you mix the plaster? It will at least help to hide any chips and it may also help any final paint covering to bond to the casting.
Carim
p.s. I think the casting system that PJ was referring to was "Linka" Yes indeed - "Linka", thanks for the reminder ("senior moment", as Nev would say  )
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1 user liked this useful post by PJMärklin
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Joined: 25/09/2006(UTC) Posts: 945 Location: Sydney, NSW
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Hi Eric/All, I haven't experienced large areas where the plaster has a glossy surface and the acrylic paint won't soak in to the plaster. I'm not sure what type of plaster you are using but I use casting plaster bought from CSR building supplies. Don't use too much water in your plaster mix. It should be creamy and not a runny mix. For those interested see my article on casting wallsLink corrected 01-01-2025 For the mould release I use Helmar food grade silicon spray. Hope this helps Edited by user 01 January 2025 04:27:59(UTC)
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Ross
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2 users liked this useful post by Ross
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 Joined: 15/10/2006(UTC) Posts: 2,319 Location: Washington, Pacific Northwest
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Assuming that its not a differences in the plaster itself, I'd think a mould release agent, or something left in the mould from a prior use, could be what is blocking absorption on the cast parts. For the ones that don't take water-color paint, what about when you apply that water-color to the backside of the pieces? Those surfaces would not be exposed to the release agent or the mould material itself and provide a better indication of the nature of just the plaster. |
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3 users liked this useful post by Minok
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 Joined: 08/11/2005(UTC) Posts: 3,528 Location: Mullerup, 4200 Slagelse
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 I also think it's the release agent When making plasterboard for bathrooms, silicone is mixed into the plaster to prevent it from absorbing water. I used to work at a plasterboard factory; but I'm not an expert https://www.marklin-user...-gypsum-and-plasterboardPer. |
If you can dream it, you can do it! I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. In case this is not legally possible: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.  |
2 users liked this useful post by Purellum
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Originally Posted by: Minok For the ones that don't take water-color paint, what about when you apply that water-color to the backside of the pieces? Those surfaces would not be exposed to the release agent or the mould material itself and provide a better indication of the nature of just the plaster. Yes I did actually test this when I did the photos and found the paint soaked in fine on the back, indicating something on the front causing the problem - which would mean the flow agent. I forgot to put that in my original post and I think others would have sooner considered the flow agent to be the culprit. I guess my reluctance to accept it was the flow agent is that I was sure I had tried casts without the agent and found the same problem. Also, that the instructions say to use the included flow agent and then speak about the plaster soaking the paint which indicates it should not be a problem. Well regardless I went and did some new casts last night with no flow agent and a thicker plaster mix, so I will paint them this weekend and see if they are absorbent or not. I will probably re-cast all my platform pieces but I have about 20 tunnel wall casts which I don't want to throw out and since they aren't seen much I am going to try using a primer first and see how they paint after. As mentioned, I can always use standard acrylics on those where the porosity of the plaster is not a concern - I just won't get the finish I had aimed for. I had a look at using some lightweight hydrocal but WS want $30 for ~1kg of the stuff when for $5 I can buy 2.5kg of Plaster of Paris. They say it goes twice as far as regular plaster and it probably does but it's still expensive. I may use it (or perhaps dental plaster) for platforms and thinner casts though for the added strength. |
modelling era IIIa (1951-1955) Germany |
1 user liked this useful post by applor
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Originally Posted by: Ross Hi Eric/All,
I'm not sure what type of plaster you are using but I use casting plaster bought from CSR building supplies.
Don't use too much water in your plaster mix. It should be creamy and not a runny mix.
I have been using Dingo Plaster of Paris from bunnings, 2.5kg for $5. Very economical. I am interested in your ratios for mixing water:plaster. I have read that between 1:3 to 1:2 is normal but even at 1:2 I find the mix so thick I cannot pour it and have to spatula it into the mould. Spoerle says to do 3:5 which is what I have typically been doing or maybe a bit wetter. My early casts always had a lot of air bubbles which is why I went runnier, though I am a lot better at casting since then and was just given a new technique where you half fill the mould and then drop it ~5cm which really bangs out the bubbles - so I will give that a try too. |
modelling era IIIa (1951-1955) Germany |
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Joined: 25/09/2006(UTC) Posts: 945 Location: Sydney, NSW
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Hi Eric/All, The Plaster of Paris shouldn't have any additives to make it more workable etc. As for the ratios of water and plaster I use about 1:2.5 I haven't measured it exactly but I use two containers of the same size to get the mixing proportions about right. From what you have stated about the amount of water you use it is far too much. How do you mix the plaster? Do you add the water to the plaster or do you add the plaster to the water? If you add the plaster to the water you will find it easier to mix to get a nice creamy mix with less water content. Hope this helps. applor: wrote:
I have been using Dingo Plaster of Paris from bunnings, 2.5kg for $5. Very economical.
I am interested in your ratios for mixing water:plaster. I have read that between 1:3 to 1:2 is normal but even at 1:2 I find the mix so thick I cannot pour it and have to spatula it into the mould.
Spoerle says to do 3:5 which is what I have typically been doing or maybe a bit wetter.
My early casts always had a lot of air bubbles which is why I went runnier, though I am a lot better at casting since then and was just given a new technique where you half fill the mould and then drop it ~5cm which really bangs out the bubbles - so I will give that a try too.
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Ross
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Originally Posted by: Ross Hi Eric/All,
The Plaster of Paris shouldn't have any additives to make it more workable etc.
As for the ratios of water and plaster I use about 1:2.5
I haven't measured it exactly but I use two containers of the same size to get the mixing proportions about right.
From what you have stated about the amount of water you use it is far too much.
How do you mix the plaster? Do you add the water to the plaster or do you add the plaster to the water?
If you add the plaster to the water you will find it easier to mix to get a nice creamy mix with less water content.
Hope this helps.
I add plaster to water slowly through a sieve and allow it to sit for a bit before mixing. It is the correct method and gives me no problems. I use measuring cups with a jug, so I normally fill the jug with 2/3 cup of water and then add 1 cup of plaster. It mixes into a thick cream which I can pour into the mould. If I were to do your ratio of 1 cup water to 2.5 cups of plaster I would not even be able to mix it, it would be too thick. I don't use any additives in my plaster. I spray a little flow agent into the mould, brush it around for an even coating and then bang the mould to ensure there is no excess. That is the exact process provided from the supplier. |
modelling era IIIa (1951-1955) Germany |
1 user liked this useful post by applor
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Joined: 25/09/2006(UTC) Posts: 945 Location: Sydney, NSW
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Hi Eric,
It sounds that you have the correct technique.
My measure is by the look and feel of the mix.
Keep practicing and enjoy yourself.
Regards
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Ross
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Just tried painting a cast from over a week ago with no wetting agent. It was still resisting my water paints. I tried on the back and found the same level of resistance. The way I see it, it is either my plaster:water ratio or the cure time. I have read it recommended they cure for a month. It may be the week was not long enough and could try speeding the cure in an oven I have been painting my other casts with 'normal' paints and it's gone quite well. Edited by user 15 June 2019 05:29:36(UTC)
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modelling era IIIa (1951-1955) Germany |
2 users liked this useful post by applor
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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On the left we have my two casts that painted great, the rest are all over washed because they wouldn't stain properly.  Since the problem is not the wetting agent, that leaves the plaster - something I ruled out earlier since I have done casts that painted well. A Stummi member however has come across my problem and found the plaster was the cause - different batch qualities despite being the same brand etc. Moving forward then it seems I have two options: 1) try a stronger WS paint blend, or mix with matt medium so that it paints on the surfaces instead of washing into the cracks 2) purchase a different casting compound - a shop nearby sells hydrocal for a reasonable price. Quite possibly will resolve my problem but theres a chance it won't. I could buy WS lightweight hydrocal which would work but its just so expensive here. |
modelling era IIIa (1951-1955) Germany |
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Joined: 15/10/2006(UTC) Posts: 2,319 Location: Washington, Pacific Northwest
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So the ones that took the water color and the ones that didn't take the water color, the plaster itself came from different batches (bags) of plaster? Interesting.
On hydrocal, my instinct is its fibrous nature would make it unsuitable for using in molds with fine detail, so I'm looking forward to hear how that experiment turns out. |
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 Joined: 03/08/2011(UTC) Posts: 1,571
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You could try and buy a small batch of higher quality casting plaster, use similar method and paint you have.
I suspect your conclusion is correct; impurity’s in the plaster you are using.
Personally I never had issues with WS lightweight hydrocal casting plaster and it takes pigments very well. But yes normal cheap plaster of Paris I used on my landscaping for non casting purposes will take colours differently and here I normally use acrylic colours instead of pigments or diluted paint.
Good luck!
Best Regards
Lasse |
Digital 11m2 layout / C (M&K) tracks / Era IV / CS3 60226 / Train Controller Gold 9 with 4D sound. Mainly Danish and German Locomotives. |
1 user liked this useful post by Danlake
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Well I am just back from a brief 4 day holiday. I have done a number of tests though. First I tried painting a platform with my water colours in a higher concentrate, I think I did 1:4 instead of the standard 1:16. It did not come out well. Richer colour but still patchy and spotty due to different amounts of paint being absorbed. This is shown in the middle. Second I tried painting the platform with my tamiya acrylics at full strength. Too rich and an uneven finish. I think they are too pigmented and too runny, rather than cheap acrylics that have a heavy use of binder. This is shown on the left. Right platform is for reference, my original stain that come out correctly.  After returning from my holiday I tried staining a rock face plaster case I did a couple weeks ago. Perfect staining with my water colours, as it should be. This is actually a new mould and no flow agent used:  So with that in mind, I decided to try normal staining on a platform I had cast with not only the exact same bag of plaster but also the same mixed batch and with no flow agent used. As you can see below in the top picture, it hardly took any paint at all! So why did it fail but the rocks did not? Perhaps the previous repeated uses of flow agent have altered the mould? I also tried using a primer/sealer and then water colours. I did the top half with sealer and then normal 1:16 watercolour stain. Way too weak, you can't even see the colour now. The middle half I used my 1:4 watercolour stain. It come out alright though the brush texture of the primer shows. It might be an option.  My last option is to buy a bag of hydrocal and cast a platform to see if it stains with water colours or not. Otherwise my options are to either use a primer or buy new acrylic paints that won't be affected by the plasters porosity. |
modelling era IIIa (1951-1955) Germany |
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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A few days ago I washed out all my moulds with warm soapy water that had been used with Lukas flow agent. I then did a test batch and last night they were cured enough for me to paint. The results:  As you can see cleaning the moulds was successful for the most part. There are some patches and areas that were still sealed but another more thorough clean will fix that. It is quite striking the areas that would not stain. After the success with staining the platform I then painted the other casts where the moulds had been cleaned and experimented with colours:  Base coat for red brick  So in conclusion I am very pleased that the cause and solution has been found - but it has cost me months of work. Thankyou all for your assistance in identifying this problem and by chance if someone is using water colour stains then hopefully they will see this and avoid the same trouble I had. From now on I will avoid the Lukas wetting agent and use only pure alcohol. |
modelling era IIIa (1951-1955) Germany |
2 users liked this useful post by applor
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Joined: 15/10/2006(UTC) Posts: 2,319 Location: Washington, Pacific Northwest
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Cool. Glad it’s solved. For flow agent would not vibrating the filled moulds on a shaker table solve the issues of flow without having to put an additive in the moulds? |
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Joined: 21/05/2004(UTC) Posts: 1,768 Location: Brisbane, Queensland
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Originally Posted by: Minok Cool. Glad it’s solved. For flow agent would not vibrating the filled moulds on a shaker table solve the issues of flow without having to put an additive in the moulds? Not entirely, I have had varied success with every shake/lift/drop technique you can think of. I can probably manage without a flow agent using a runnier mix and being careful but you get one badly placed air bubble and its a write off so would prefer to use alcohol as well. |
modelling era IIIa (1951-1955) Germany |
1 user liked this useful post by applor
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Joined: 23/07/2014(UTC) Posts: 8,472 Location: ENGLAND, Didcot
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Originally Posted by: Minok Cool. Glad it’s solved. For flow agent would not vibrating the filled moulds on a shaker table solve the issues of flow without having to put an additive in the moulds? It would be better to vacuum the air out. I would put it in a box that I could connect a vacuum cleaner to and see if that produces enough suction to pull the bubbles of air out of the plaster.
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Joined: 15/10/2006(UTC) Posts: 2,319 Location: Washington, Pacific Northwest
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Originally Posted by: kiwiAlan Originally Posted by: Minok Cool. Glad it’s solved. For flow agent would not vibrating the filled moulds on a shaker table solve the issues of flow without having to put an additive in the moulds? It would be better to vacuum the air out. I would put it in a box that I could connect a vacuum cleaner to and see if that produces enough suction to pull the bubbles of air out of the plaster. Interesting concept.... I've got a vacuum sealer, and I think it has a hose/port for suction, so I could use that and connect it to a container I can seal the mold in and decrease the pressure. Maybe get a nice plastic vacuum box for it. Or a simple " space bag" in some small size, though those would require more cleaning. |
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Joined: 23/07/2014(UTC) Posts: 8,472 Location: ENGLAND, Didcot
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Originally Posted by: Minok Originally Posted by: kiwiAlan Originally Posted by: Minok Cool. Glad it’s solved. For flow agent would not vibrating the filled moulds on a shaker table solve the issues of flow without having to put an additive in the moulds? It would be better to vacuum the air out. I would put it in a box that I could connect a vacuum cleaner to and see if that produces enough suction to pull the bubbles of air out of the plaster. Interesting concept.... I've got a vacuum sealer, and I think it has a hose/port for suction, so I could use that and connect it to a container I can seal the mold in and decrease the pressure. Maybe get a nice plastic vacuum box for it. Or a simple " space bag" in some small size, though those would require more cleaning. Biggest problem I see is that you probably need a box that won't collapse when under very low air pressure. The vacuum bag sealers tend to not worry that the bag is collapsing, but I'm not sure you really want the bag collapsing onto the wet plaster, so it probably needs some form of box that won't collapse. The other thing I forgot to say is that as the air pressure falls in the box the plaster will foam up and possibly overflow the mould (in the past I have done this when outgassing epoxies we have used). I would try partially filling the mould before vacuuming it and using a very sloppy plaster mix. The mould can be topped up afterwards when the initial mix has partially set.
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Joined: 15/10/2006(UTC) Posts: 2,319 Location: Washington, Pacific Northwest
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That’s what the rigid box is for.  I already happen to have a bag vacuum sealer that has a hose port so with the box and hose adapter I’ll have a rigid box to put plaster under a vacuum. It will be some years before I get to the stage where I’ll be casting rock molds to try it out though. FoodSaver Vacuum Sealed Fresh Container, 10 Cup, Clear - FAC10-000 https://www.amazon.com/d...r_cp_api_i_J3qiDbNW3AG5J |
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