No, I just cleaned it only with alcohol before spraying. If I remember correctly, that is all they recommend for prep is to ensure surface is clean.
Very nice Maarten.
Well yeah, it kinda depends on the paint you're gonna use what's best preparation. If you use proper paint that is actually designed for the type of plastic your'e gonna use it on then chances are there's solvents inside the paint that actually soften the plastic a tiny bit as you spray it on guaranteeing a proper bond between the plastic and paint when the solvents evaporate. In that ideal situation removing fat and dirt are your main goals, hence alcohol will work fine indeed.
I however used CHEAP paint, not really designed with any specific plastic in mind so i needed surface area for more stickyness power, hence the sanding and as with any sanding job ALWAYS clean your subject before AND after the actual sanding (sanding a dirty and or fat surface will actually work the junk deeper inside the material making it impossible to remove afterwards).
One way is not per-se better than the other, it all depends on the materials you use really.