I want to start the body work on by 85 Camaro z28 soon, however, I am just not sure how to sand/prep the front and rear bumper, ground effects, and spoiler for paint. I have helped sand a car previous and have a pretty good idea on what I am doing! However, I am about clueless when it comes to preping these plastic parts for paint.

My car has never seen salt and the body is in excellent condition with the exception of a textural difference in the front bumper ground effect, clear coat peeling, and I also have a chip in my stock spoiler. So basically I am just curious to what kind of sandpaper I should be using for the plastic parts of the vehicle and what I can do to prevent possible cracking in the plastic after paint?

Thanks,

Zach

 

https://www.thirdgen.org/forums/body...aint-what.html
https://www.thirdgen.org/forums/body...er-covers.html
there should be enough info yo help you out

 

If you sand down to bare plastic, use Bulldog adhesion promoter. I just finished repainting the nose of my car and used Bulldog...works great. As far as stripping...I used 80 grit on a DA. The plastic will fuzz up a bit but you can knock it back down with finer grits.

 

It all depends on whether your going to prime them or just paint. If you are going to prime use 180grit DA. If you arent going to prime you have two options 1) sand it with 320grit DA or 2) Use a red scuff pad and make sure you have all the shine off. I recommend the first option if you are just painting. You prime over 180 and paint over 320. If you prime then after wet sand it with 600grit wet sanding paper then either seal or paint

 

good advises but you guys are forgetting the flex agent to the paint to prevent cracking thats why i gave him the links.....

bulldog works but the way you xplaned 406tpi it is just for the paint to stick to the plastic

also if its gonna be primered it has to be a plastic primer not just any primer otherwise it'll flake off eventually

 

The painters Ive talked to said that a flex agent isnt neccesary, I guess it wouldnt hurt to put it on there though

 

that's exactly what the guy that painted my friends camaro said and after 3 months the paint on the front and rear bumpers started to show the crack like strands again... just like i was talking to an old timer here on TGO the other day, the guys that do bodywork and pain now days don't wanna take the time nor effort of taking extra precautions because they're into production and not quality ans longevity.
(https://www.thirdgen.org/forums/body...o-i-paint.html) post #44

 

I thought the flex agent is only good for 15 minutes or so to allow you to bolt the piece on the car. After that it supposedly hardens just like the regular paint.

 

Your paint tech sheets will tell you if you need flex additive, most don't require it.

 

I owned my own shop and before that did bumper refinishing, so I know what works and what doesn't. flex agent doen't do anything of any benifit that I have ever seen. With plastic the key is not to use anything thick and to stay in your tech sheet's specs for thickness. Cracking and lift type peeling is because of layer adhesion. With an adhesion promoter you need to to an exercise in logic. Bull dog is sprayed on before base coat after priming. when was the last time you saw bare primer showing through a pealing base coat? Paint only has the ability to adhear to a chemicaly wet surface or a mechanical bond. Bul dog takes your mechanical surface and makes it a chemical one, for this primer sealer does much better. Aftermarket bumpers have a water based primer most of the time and this has to be preped, I never would suggest base coating any surface you did not apply with 320 and base coating. Most bumpers that are oem have been sprayed many many times and that thickness has to go first mine had been coated over 20 times. The biggest factors you need to pay attention to with a bumper are buy two body style stands to hold it best, Preping a bumper right should take you more time than preping a entire side of a car. I worked for an oem rare bumper remanufacturer, I was a heavy repair tech and had 2 prepers and 2 primer finishers working under me. A production preper took 2 hours to fully clean and prep-sand a entire bumper. It is very tedious work to hit every contour and get into every crevice. I would never start with anything finer than 220, cleaning and sanding everything takes a lot. you need to remove your front spoiler to get behind it and you need to really pay attention to quality primer, and a proven clear coat that can handle the flex of a bumper. some clears that work well on a metal panel chip way to easy on a platic piece. You need a clear that doesn't get too hard because that is the reason for most rock chips, that and clear coat beeing too thin.

 

thats basically what the person that did my paint job told me and his specialty is classic restoration meant to last not mass turn arounds in a typical insurance/maaco shop

 

just as an example, i work with this on a daily basis, once we remove this molding buh bye paint it cracks and flakes off from factory... our solution add flex agent to the paint if we remove it again for whatever reason we don't need to pain it again paint doesnt crack nor flakes off (proven many many times on $700,000 motor homes, and not only on those moldings but on many other plastic parts this couches use.


(also, bull dog is a low quality product compared to other flex agents and adhesion promoters )

 

I'll take your advice on the plastic primer.......would you consider SEM flexible primer for plastics/fiberglass etc. (SEM#39134) from TCPGLOBAL the right type of product to use?

 

Yup, i've ben using that product for years no problems.its kinda hard to sand but its better than having to re do your work for using the wrong primer.