I am sorry, but where do you get this stuff? Remove decals where you are going to paint with WD-40? Not anywhere anything I am painting! A wax and grease remover, a urethane reducer, even lacquer thinner but certainly not WD-40! HVLP gun works well with smaller compressors? HUH? High VOLUME
, small compressors don't have high volume, they have very LOW volume, HVLP needs a BIG compressor that produces HIGH volume.
Retarder? RETARDER, he is painting a tiny part that requires only a couple of passes a foot long, why in the world would you want a retarder?
I agree that a build up of paint causes easy chipping but we need to look at the "Bestest" way sometimes. Stripping plastic can really be a chore, it often results in marred up plastic that then needs to be primed and surfaced, building up material again. For a newbe, painting over the clear and decals may be the best way to go. But yeah, if one of these parts were stripped to see how it goes, you are right stripping is the best way to go. But if stripping that one panel gets you into trouble, maybe sanding and shooting is the way to go.
Even that "wonder wheel" is often too aggressive for a plastic part and will "grind" plastic away. A lot has to be thought about before stripping plastic with multi layers of paint and decal. It can be very difficult proving disastrous to a newbe.