If you are talking about tiny areas you might get away it with retouching but it often shows and looks worse than leaving it original. If the missing bits are big is going to look terrible no matter how much care you take. In that case only full stripping, and respray would do which will include getting new decals. Probably someone else would volunteer some brand names for the paint but if you think your previous attempts were not successful you are just setting yourself for deja vu