If you mean a flat-painted surface when you say "rough", there's an easier way. Coat the surface where the decal will go with several coats of "Future Floor Wax" until you have a smooth surface. You can shoot Future on straight without thinning from an airbrush, just keep the coats light to prevent running. This also makes the decal slide around easier for final positioning and keeps them from breaking. Wait about 24 hours after the last coat of Future before applying the decal. Once the decal is on and set, re-coat the area with Dull Coat and it will blend with the flat painted surfaces around it. The Dull Coat acts as a sealer for the decal and helps protect it as well as keep it from yellowing with age. If by "rough" you mean a surface with alot of detail, the same process helps give the area smoother contours and easier decal placement. Give it a whirl.
The first time I ever saw a jet, I shot it down.
— General Chuck Yeager, USAF, describing his first confrontation with a Me262