Decals are ok to clear coat but take a little more care when shooting.

It sounds like you are working with bodywork that has already been clear coated. In that case if you're using P rated paper 800 is a little fine and could lead to adhesion problems. I would use P600 wet with a lot of water and feel whats going on under your paper. Use quality paper 3M or better, it does make a difference. A good way to check your work is sand it and then run some water over it and look really closely, that's how it will look when it's clear coated.

If you are placing new decals on every panel then wet sand the entire panel. If there are existing decals on the panels try not and break through the clear on the edges of those decals. You can do what's called a blend on just the places you are placing new decals but it's harder to get a nice job that way. Much, much easier to shoot the entire panel with clear and be done with it.

Good luck.

Edit: One thing I forgot to tell you since I always do it without thinking is, clean the panels with wax and grease remover or at the very least a strong soap like Dawn before you wet sand.