Uggghhh!!! Not to be a pessimist but you used THE all time worst thing other than Armor All that most bodymen freak out about!!!! What type of material is it? Plastic I know but is it a hard plastic, flexible? Has it been sanded? (Let's pray not) OK, now that I'm over my light panic attack, I am going to pretend this is a new bumper cover and I'll follow through what I do to prep for paint. First and foremost clean it, I normally like to use Dawn, but nothing with lotion or junk like that, just plain dishwashing soap. Next I use a prep solvent of some sort (enter brand name here lol). After that has enough time to dry or blow dry it is time to start sanding. Hopefully the material you are painting is not porous because if you have let WD40 sit on there for an amount of time it has soaked in and from there on out let the fun times begin trying to keep from getting fisheyes.
As for what type of paint, are you meaning rattle can or automotive?