What kind of paint should I use on my plastic mudguards?
My first step would be to scuff the surface with a Scotch Brite pad. You want some minor surface imperfections for the paint to adhere to. Clean the surface with either alcohol or what the instructions for the particular paint call for. I am partial to automotive brake cleaner. If you can get it, use Rustoleum Spray specifically for plastic. Apply two coats of primer. Allow it to dry between coats. Then apply two color coats. You have to use a paint intended for plastic because conventional paint will not flex without cracking or chipping . As @Daniel R Hicks has stated, allow the paint to fully cure before it experiences any contacts. If you can set it in the sun for a couple of days it should excelerate the hardening. Even doing this the paint may chip if the fender experiences severe flexing.