i have a lot of problems like that with zbrush also, i think you need to put a clear texture on before you paint in order to recover it.
the one thing that pisses me off about zbrush is that it feels like an amature started it and someone just started adding things that everyone can use after the fact.
Usefull, none the less though. cant live without it now. Displacement maps have never been so good.
And after calming me down with some orange slices and some fetal spooning,
E.T. revealed to me his singular purpose.
--TOOL, 10,000 Days---