[It's Just Eric]

Ahem, Ill step in here

Question 1:
Are the plastic parts new or used?
If they are new, the procedure is simple, but the materials needed are application specific. If they are used, you have a TON of work ahead of you

IF they are new, you need a special cleaning agent to remove the mold release from the part. Mold release is what keeps the plastic parts from sticking to the mold they are made in. Its also super good at preventing paint from sticking. My reccomendation: start the process by flambe'ing the part with a small propane torch, then, use a special cleaning agent in conjunction with a scuff pad (Please, contact your local autobody supply store for specific details on the products they offer) to not only clean, but fully scuff the surface of the part. Dont forget the corners! youll know its done when it has NO shiney spots. Flambe it again for good measure, then spray with a flex agent. From there, you should be able to seal it, and base/clear it

IF the part is used, foret everything I just said
firstly, assess the condion of the part. Look REAL close at them stone chips, scratches, ect. if it is a bumper, check the paint for spider web like cracks, which indicate hitting things like parking posts and whatnot. Run your hand along theese areas and feel for high spots

Step one: heat gun and push down any high spots. If this doesnt work, get a new part. It isnt worth repainting the car an having a lumpy mess
Step two: bust out some sand paper. Genral rule of thumb, use 400 grit. If it's 3m paper (it likely will be) note that A and P are diffrent ratings. 400a is what yu want...400p is oing to be way too fine. Easy way to make sure you got it right, the a grit is black on yellowish paper, P is grey on brown
If theres alot of chips and damage, you'll likely want to go down to 220 grit to make sanding go faster. after getting it MOSTLY down, finish with 400. When sanding, use loooong strokes and a block to avoid making waves. Dont sand just one spot out. Remember, your not digging the chips out, your lowering everything around them to their level.

Odds are, by the time your part is sanded CORRECTLY, youll be looking at a mostly bare plastic part. If there are NO break throughs, skip the next step

clean the part with a rag and some rubbing alchohol. guess what you have to do? Nope, dont worry about special plastic cleaners, silly. the part has paint on it, so the mold relese has obviously been taken care of. *note, if the part you have needed to be repainted because most of the paint flaked off,then yes, clean it as per my first paragraph*
All you gotta do is spray the exposed plastic elements with the flexible primer adhesion promoter stuff (Once again, ask your body shop equipment supply company for specifics about the products youll be using)

Now your ready for primer. Odds are, some of the scratches and whatnot went slightly into the plastic...so your going to want to lay the primer on thick. for example, I had parts that were pretty rough...still had a few scraches and whatnot..I layed on 4 HEAVY coats of high build primer.

No, you cant use primer from a rattle can

BUT, after primer, youll want to dust on a very thin layer of spray pint, any color that stands out from the primer. See next step

Next step:after primer, your ready for finishing sanding. Once again, you'll be using 400 grit, a block, and long strokes. This time time though, the object is to get every bit of texture out of that primer you just bombed on. you'll notice how much better fresh primer sands than 15 year old paint as well. Wet sand the part in question untill all the spray paint you dusted on is gone. Yes, look at the part on a microscopic level to find imperfections, otherwise they WILL show. Also, and this is just to be an absolute perfectionist, finish the sanding off with 600 grit (Or 400p, which is acually abou the same as 700a...um, nevermind,ignore this, its just causing confusion)

now the part is ready to be sealed, then base / clear

if you have any questions involving anything related to anything having to do with anything about bodywork, but only regarding your specific project, shoot me a pm anyime between 9am and 11:37 pm on monday -thursday, 10:03 am - 10:05 am on fridays, and anytime saturdays, with free texting and roaming on sundays

-Eric