No matter where your box is you allways have to worry about moisture, it's in the air all the time, even in doors.

For painting, the inside a speaker box, I'd sand the primer with 600Grit (after all the coats), then sand the color with 1000 -1500 Grit (1500 can be hard to find some places) before the clear. Then just lay the clear on and don't worry about sanding it (as no one should see it, and if they do the light will be so low that the clear's shine will be wnough to look smooth.


Good Luck!
Crazybillybob