You should clean the parts down to the base plastic. I don't want to start anything here, but Brian is right about WD-40. Why don't you check with anybody who paints for a living and see if they even allow a can of WD-40 in their shop?
Any glue residue from a decal can be removed with mineral spirits or Xylol. Plastic parts can also be painted with products intended for plastic, and should include an adhesion promoter to draw the color into the plastic. Check out SEM products. CLICK HERE
They have coatings in aerosol form that will give you a perfect outcome on plastic.
Originally Posted by snydski
Build up of paint causes shelling or chipping I say Strip it . The use of toluene paint thinner will blister the clear coat after a few seconds, and then can be scraped off with razor blade , if you don't get it the first time do it again you will definitely feel the paint getting soft . It is less caustic than paint remover which will work , but might melt the plastic . Then take a heat gun to the decal or use the "Wonder Wheel" as seen on TV to remove the decal. Remove any residue with W-D 40 and wipe with denatured alcohol. I would use a "Two Part" catalyzed polyurethane and retarder with recommended thinner most likely methyl ethyl ketone , but not too thin this paint is made for .0003 mils a coat , , wet coats that are applied with a HVLP (High volume low pressure) sprayer which works well with smaller compressors and these paints don't need a clear coat but they must be mixed well and strained . If you live by a Marina these coating are used for boats and bath tubs and even tile the other stuff can be got at lowes. The mix that worked for me was 2 part base to 1 part catalyst and thinned 25% by volume not weight . paint takes hours to dry and 3 days to cure but it cures long after that.