I have never done any hydro dipping but have had similar experiences with watercolor paint.
What kind of clear coat spray or brush on? No doubt the hydro dip comes off with 50-100 grit that is very coarse for delicate work and will quickly cut through a thin clear coat. I would try super fine like 800 grit up. I use automotive sand paper and I don’t wet sand it unless I need to take a lot off. I would only do very light sanding just enough to put light scuffs in the clear. They don’t need to be extremely visable even if it is enough to show some light dust on the sand paper will at least give the spar or epoxy to hold on to.
I am also beginner in hydro dipping, basically just tried dipping my helmet and two enclosures for my first build with somewhat mixed results (enclosures depicted below)
If I understood correctly you think it should be enough to use very fine sand paper like 800 to just slightly roughen up the clear coat layer in order for the epoxy or spar to stick or hold on to?
nope the spar is used in place of the epoxy and vise versa depending on what you have on hand and how quick you need to do boards. I use Spar just like @longhairedboy because when I’m doing a board I want to be able to do a few coats a day where Epoxy takes longer to dry.