Hi Paul, what would you use to smooth them over once the backs are heated up?The wet & dry is the best method (circa 1200 grit or higher) just to take the edge off the scratches. I would also lightly run over the seat backs with a heat gun (or hair drier at a push) to soften the plastic, and level it off. It will more than likely make the surface shiny, so that's why I'm saying to do them in entireity (so it doesn't stand out). You may not need to colour it this way, as the plastic is (usually) coloured all the way through - any difference you are seeing just now is temporary.
Hope that helps