Re: Casein -> it's the paint
Anyway, there are no contra-indications on the use of an excess ammonia, because it will leave the coating while drying.Perhaps Kremer's casein is easier to dissolve and/or slightly different than what you were using.
Casein could be dissolved also in other alkalies, e.g. borax or NaOH, but in this case their amount has to be carefully weighted. There are some patents in this regard, because casein dichromate (very insoluble in acids) was used as a resist for the etching of colour TV screens.