A satin finish is smooth enough,,,,,,,think of decals being okay to go on a glass smooth surface (gloss), and a pebble smooth surface (Satin, or Semi-Gloss), but, not on a gravel surface (Flat, Matt, or Non-Specular)
the same process that lets a decal snuggle into detail and panel lines lets it snuggle around the "pebbles" of a Satin finish, because there is no air trapped under the decal as there is with a Flat
Rex