Do not quote me in any of this because I have yet to actually try it for myself, but this is the general sequence I have heard works well, and will be trying myself:
-general assembly, filling/sanding, and having dust carefully washed off.
-paint (preferably useing a gloss paint as decals stick to a gloss surface better, and are less likely to silver. if you use a flat paint, it is generaly recomended to then spray a gloss clear coat over it. if you use a gloss paint from the get-go, you can skip this step)
-decal (you can use whatever setting solution you like; I am cheap however, and dont use any.)
-spray a final clear cote (this does two things; a- it is the finish you want(gloss, satin, flat, etc.) and it also helps seal the decals on, so that if they did not adhere well, the final coat will help to seal them on.
Not sure if this will help you or not, but it is the order I have heard works well, and I trust it enough to be willing to try it for myself.