Satin, semi-flat, semi-gloss are all pretty much the same. On suspension parts, I always sandblast, prime with PPG epoxy primer, & paint with semi-flat black urethane. On the spaceframe, if everything is smooth you only need to sand it well, & you don't HAVE to prime it - but I prime with the epoxy as it gives maximum adhesion.
Manufacturers just use whatever paint they can get the cheapest at the time...
aka "Tha Driver"Custom Fiberglass Parts