I am a professional automotive painter and would like to share a few tips on painting plastic/rubber items.
As far as "roughing up" raw un-primed/ un-painted plastics, you need to be carefull about how coarse of a scuff pad you use. On raw plastics using a Red Scotch pad will often cause some deeper scratches than normal when used on raw plastic that will cause a type of burr on the edges of the deeper scratches. This can show up depending on how thick of a plastic primer surfacer you use. Grey scuff pads work fairly well with a much lower amount of deeper scratches standing up through the paint and clear coat. Gold works very very well in this regard, it will dull the surface of the raw plastic without causing any of the deeper "gouges" that will show through your paint. These gouges are not very visible unless you are using a metallic color paint. Solid colors hide these imperfections very well.
Most automotive paint company's or parts stores that carry automotive paint will carry a cleaning agent in the form of a paste designed to be used during the scuffing process and then cleaned off with water and a cloth after scuffing. These cleaning paste's are designed to be used on raw plastic bumpers,mouldings,side-view mirror's,cladding's,wheel opening mouldings etc.
Simply wet the part to be sanded with a spray bottle or water hose depending on how big of an item it is. This paste remove's any mold release agent that may be present on the raw plastic items. Simply squirt some paste on your scuff pad, moisten the pad and part to be scuffed/cleaned, and lightly abrade the surface while trying not to "dig in" too much causing un-needed deep scratches in the plastic. You are trying to scuff the piece not "scratch" it if that makes sense. This is especially important when you are trying to achieve a very high quality finish that is perfect with no flaws visible even when the item is clear coated after painting, and doubly critical if you are using paint with a high metallic content such as many automotive colors. The coarser the metallic the more sensitive to deep scratches the finish will be. When done rinse with water. And clean immediatly prior to painting with alchohol or acetone will work as well.
This can be mediated somewhat by using a proper plastic sealer that has some "build" to it. It functions like an "adhesion promoter" such as bulldog but has the added benefit of having a bit of build to it when applied in multiple coats. *usually no more than 2* This allows you to achieve a couple of mil's of build on an item and sort of hides some of the scratches or any minor imperfections that may be present in the raw plastic part from the factory. I use this on every single raw plastic item i paint. However perfection is the goal in my case and a single scratch showing up on a raw plastic part after painting means I have to re-sand and re-paint, not something I have time to do.
Using a plastic primer surfacer, the proper plastic cleaning paste, and proper wipe down procedure you can insure adhesion to pretty much any type of plastic for years upon years, and it will stick through much abuse. My paint has to endure bug's, slamming into car bumpers at 70 mph so I have to do it right. All those cars you see roaming the road with paint peeling off of the bumpers were not painted using the proper adhesion promoting material's. These steps will ensure your paint does'nt "peel" off when chipped. It will still chip off if struck hard enough by a pointed or hard object but that should be the extent of the damage, no paint peeling off in sheets
P.S. You can also use a plasticiser additive in the clear coat you apply as well, it will make the clear a bit more *chip" resistant. However this will increase flash times and cure times. Watch out for them runs! Plasticising additives added to matte finish clears can change the final gloss level of the product as well so use caution.
P.S.S. If you go down to your local body shop and have them mix you a small amount of plastic primer be sure to get your tail in gear and spray it within 30 mins or so if it is a catalyzed product. If it is a 1k product *not activated with hardner* it can last for week's when properly stored but 1k products as a rule are not as durable, and are subject to "eating up" when another product is applied on top of them in too heavy of a fashion.