I don't think I saw anybody mention using a BRUSH when cleaning, not wire, aplastic bristle brush and scrub it, wipe dry (obviously with a clean, lint free rag). All the cruds, oils, protectant residue, is in the 'texture' (those little cracks and crevices) of the plastic. Sandings only gonna clean the peaks, all those valleys will be still full of it. As you'll learn the hard way, some solvents will attack, melt, dissolve, plastic so be careful what ya clean with and how long you leave it on the surface.

Satin (ie low gloss) won't give you that cheesey look on interior parts. Follow the mfr's instructions for application and recoating. Basically though, one heavy coat nearly never works, on anything....