Okay - I'm going to throw in 2 cents worth here. I've done a
little electroforming, which I think is probably similar. One
product I've used successfully as a mask is called Electro Mask
Protective Coating, made by EFS Company in Ocala, FLA.
904-873-0089. It can be thinned and the best part is that it
peels right off afterwards. I bought mine a couple of years
ago, so I have no idea how expensive it is.
You could also try asphaltum. It can be thinned and can be
applied with a brush. It cleans up with mineral spirits.