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.

Some plaster mixes have a bit of PVA glue mixed in to re- enforce it.

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"

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)

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 walls

Link 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)  | Reason: Not specified

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-plasterboard

Per.

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.

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.

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)  | Reason: Not specified

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.

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.

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.

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.