I've had pretty good results using Testor's Decal Bonder spray. Unfortunately, it is no longer being manufactured. The decal bonder was, I believe, a lacquer. It was definitely solvent-based. I have read that artist's workable fixative works, but have not tried it.

I found it best to use two or three LIGHT coats of bonder. What you're trying to do is seal the ink enough so that the water doesn't dissolve it when you wet the decal to apply it.