i have a 90 rs camaro that i bought with a berlinetta style front bumper. and i hated it. so i got an iroc z style front bumper from the junk yard that was in great shape and the lower valance i got off thirdgenranch.com. the problem is the paint on both pieces is HORRIBLE. so i need to sand them both down and paint them. but what i am asking for is what grit sand paper do i start with to get off all the old paint and what grit to use for a final sand? and is it hand sanded or do i use some type of sander?...

 

Trying to strip the paint off one of these bumpers by hand isn't very realistic, if you own an air compressor then your best bet would be to purchase a dual action air sander. It would be a great investment if you ever plan on doing body work again. I bought a nice 3M one for work a few years ago for about $180 canadian, but you could probably get a lower quality one for under a hundred.

If your just worried about cracked paint and not shape then I would strip the paint down with 180 grit. Any coarser grit is really overkill and you end up with plastic fuzzies on the surface, which you dont want. Chances are the paint on the bumper is cracked everywhere, you need to sand all the cracks out, or they will show through the new paint. This will probably require stripping the whole bumper.

Sanding the urethane with 180 will make the surface somewhat hairy, you need to sand all the fuzzys out with either 320 or 400 grit, until the surface is smooth. If you spray primer over the fuzzy surface you will see the plastic sticking up through the paint, and flooding the primer on to cover over will cause problems. A primer with a flex agent in it is required to prevent the paint from cracking when the bumper flexes.
You should really use a HVLP gun and a high build primer, although rattle cans would probably work but it would be rough, and the amount that you would need wouldn't be much better cost wise.

After you've primed and guide coated, sand with 400 on a D/A or 600 by hand until the guide coat is gone. Block sanding isn't really necessary here because there arn't any large flat surfaces. Once the bumper is finished in 600 its ready for base/clear.

That is basically it in a nutshell, fell free to ask questions or express opinions.

 

Do not use 180 on the bumper, you will never be able to sand out those scratches.
You do not need to remove all of the paint in order for the flex primer to stick, remember the paint was baked on from the factory.
Start with a 240 wet, you can use an orbital, but I use a flex block and elbow grease, sand it so it's getting through the clear coat, then move up to a 320 wet, and then prime the surface with the flexible primer (Comes in a spray bomb, usually around $12 a can)
After it is primed, do a finish sand with 400, and if it still has lines, go up to 600. When painting black, I would even say 800 wet is a good way to go.

 

You can also have it soda blasted - Check your yellow pages and call some shops up - I'm not sure what the cost is, but I'm sure it'd be well worth the money to remove the old, cracked, flaking paint completely w/o the intensive labor involved.

 

Ditto on the 400 grit. It will take a while but get a small cup or container. Fill it about 3/4 of the way with warm soapy water. Dish soap works great. Dip frequently in between sanding. Wetsanding is very effective and it wont leave deep scratches and it will take out all those little cracks. There is also quite a few threads on how to fix holes with epoxy if you have any hole or cracks. Good luck.

Kevin

 

I guess it really depends on what primer you are going to use. If you dont sand out the spider cracks in the paint and just prime over them, they will come back, guaranteed. A high build primer sprayed with a HVLP gun will fill 180 grit scratch. 3 coats, will usually even fill a 120 grit scratch. You just need to make sure that you knock down the plastic fuzz that the coarse sanding creates. Although, if you are using a spray bomb to prime it then I wouldn't sand with anything coarser than 400. The paint sprayed from an aerosol can is much, much thinner than a high build.

 

The spray bomb primer I'm talking about is 'Flex primer', you're not supposed to use a high build on urethane body panels, a body man of 30 years taught me that.
Oh yea, it's really thin, and has to be put on in several very thin coats.

 

Ok thanks guys. But I need to take all the paint off. The paint is terrible and I want it all off and there looks like some pretty tough spots so I want to use the biggest grit sand paper possible with out damaging the plastic.

 

They do make stripper for plastic or rubber bumpers. I don't care for chemical stripping. DA with 80 to take most of old paint, primer off stopping before you hit bare bumper. 180 after then epoxy sealer followed by primer with flex agent (if required). Most primers will work as long as film build is not too much. Ask for MSDS sheet on any product you buy. It gives specific info on the product.

 

May i ask why?

How would you expect to cover pinholes and sand scratches on a bumper repaired with plastic filler, with thin rattle can primer? The sikkens paint system I use at work requires an adhesion promoter sprayed on the bare plastic and a flex agent added to the primer to prevent it from cracking. I've had sikkens reps tell me this. Dupont, PPG and RM are all similar.

 

180 on a D/A and you will be there in no time. Then buzz over it with 400. 80 grit is very agressive for just stripping paint, and a beginner would likely dig low spots into the bumper. Important thing is to keep the sander moving, dont leave it in one spot. Side to side, up and down.

 

soda or plastic media...

 

Spray high build primer on a piece of plastic and bend it after it dries, you'll see why.

 

I would use a stripper like Bulldog and you will need a adhesion promotor and and you need a plastic cleaner before you apply the adhesion promotor and here is the stripper http://www.levineautoparts.com/bulflexparst.html you may find it cheaper at you local paint store

 

I would never use a chemical stripper! it brakes down the urethane if left on to long and makes it to pliable and flexible on top of that its such a pain in the *** to deal with. you would be better off sanding by hand then using a stripper. It shouldn't be used on anything but metal it can change the chemical properties of most materials I.E plastics urethane's etc ...

 

I used SEM bumper stripper without a problem. The paint is a bit over 3 years old now and shows no signs of anything bad happening.

 

I do, everyday. I work in a bodyshop. 3 coats of high build with a flex agent and an adhesion premoter should withstand just as much abuse as the original paint. That said, the 20 year old paint on the front bumper of my rs is spider cracked from one end to the other. Plastic was never designed to be painted.
Paint sprayed from an aerosol can is a one part mix. Meaning that it has no hardener, catalyst, activator, etc. It is guaranteed to be lesser quality that the two part high build.

What you are saying is not making sense, don't use the proper high build primer becuase its too thick, yet spray 15 coats of rattle can paint to build up the surface enough to be able to work with, which is the equivalent of 3 coats of high build. You can spray on as much paint on as you want. Three coats is recommended, two and you don't have enough material to work with, and four you run the risk of shrinkage.

Too much product on a surface does make it fragile. But were talking like at least 3 paintjobs before it starts to become a problem. Were talking about a bare plastic bumper. If your still that concerned about it, only do two coats, or even one, and spend forever carefully sanding it out trying not to burn though. You should check out the paint on Dodge trucks, the e-coat is so thick you can shape body panels with it. No need for filler on small door dings.

 

1st of
You PRIME over 180
and PAINT over 320.

I highly recommend using a DA sander, a cheap one from harbor freight will get the job done no problem. Because of the fact that it is a plastic bumper I dont recommend using something as coarse as 80grit. Just go ahead and use 180grit with the DA and be patient, dont be afraid to change the disk out. After you are done with that then its time to prime. After priming wet sand and paint.
One thing Ive learned is keep a "scrap" box for all ur old sand paper incase u need it later on for something small.

 

been a painter for a while now used sem flexible primer ready to spray is for plastic parts

 

I still advocate for wetsanding with 400. It worked well for me and I highly recommend it. With the water it promotes friction and, although it takes a little longer, it will be extremely smooth. Your fingertips will also be smooth. LOL

 

We painted our IROC-Z convertible black, sanded it down with 600 and you could still see scratches.. Finish sanding was 1000.

 

I agree. The finer you sand the surface the better the paint looks underneath. No one could argue that. But they were talking about hitting it with 180. 400 grit will take the cracks out with some work. Then it would even be better to hit it with 800 or 1000. The heavier the grit, the deeper the scratches, which will need to be smoothed out. Thats a nice looking convertible you have there. Was it wetsanded with 600 or dry sanded? Just curious.

 

primer will fill 180, sealer will fill 320-400, base will fill 320-400. if you sand your primer, sealer, whatever with anything over 400 you are wasting money and time but don't forget that it will clog doing it dry and yes wet is the best. If you sanded your clear coat with 600 starting off, you should find a new painter. final sand of clear coat should be 2500 or 3000 if you want good results, especially if it's black which isn't very forgiving.

 

thanks every body for the input. i am going to start sanding today and see how it goes.