Polyurethane bumpers, need advise

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PostPosted: Sat Aug 24, 2019 11:40 am
Edited because I thought this might be useful:: I have on hand summit racing epoxy and surfacer. Will be using Napa’s crossfire line base coat and most likely finish 1 clear

I keep reading conflicting information. I read to never use epoxy, always use epoxy, just use surfacer, and I’m even reading don’t use any primer at all.

I have two bumpers to choose from. The darker red bumper came off the car, original paint with spiderweb cracking and a few scuffs that appear to go all the way to bare polyurethane. The brighter red is a bumper I picked up used that appears to be a re spray with bubbling issues and spiderweb cracking. I don’t know if either bumpers cracking is in the bumpers itself or just the paint.

With either of these bumpers I think I would have to go to bare plastic in places so I’m aware I need adhesion promoter which I’ve already bought. (Bulldog) would it be safer to completely strip all paint and primer so that the bulldog doesn’t cause issues when I apply primer/base? I’m afraid if it touches any remaining primer/paint it will cause a reaction when I spray over it.

What is the correct process? My plan as of now is:

Repair damaged areas
“Spot” adhesion promote
Spray surfacer
Color/clear

For rest of the car I’ll be applying base coat directly over blocked surfacer so I would think I would want the bumper to be the same shade as the rest of the car before applying final color.
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Last edited by Billgluckman on Sat Aug 24, 2019 11:46 am, edited 1 time in total.



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PostPosted: Sat Aug 24, 2019 11:43 am
I also forgot to mention another worry I have. This is an on going project that I’m working on little by little. As flimsy as these bumpers are I’m afraid they’ll crack again either by moving them around or even by the fact that they’ve been off the car so long they could have possibly warped.

Basically, these bumpers make me super nervous. Lol



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PostPosted: Sat Aug 24, 2019 1:16 pm
This is the rear bumper. It’s in a lot worse shape. Cracks all over that are apparently not just in the paint. Is this even fixable? The entire bumper is like this. These aren’t as easy to find used as they used to be and the ones I have found are in pretty bad shape.
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PostPosted: Sun Aug 25, 2019 1:53 am
I'd use the bar that has original paint on it. Less likely to find any little "surprises" under the paint on that one.

There is no real need to take all the paint off, but certainly remove where there is any kind of cracking or other damage.

Finish with P240-P400 and apply the Adhesion Promoter if you feel you must. I do bar repairs every day and just use a two pack primer over bare, sanded plastic. Adhesion Promoter is really only necessary where you have new, very smooth plastic and there may be a question about paint adhering to it and it is not intended to prime, or maybe just use a surfacer. Epoxy primer, mixed as a surfacer, is what I use if I don't want to sand the surface. Once sanded there is plenty of mechanical adhesion.

Adhesion Promoter is designed to be put on in one or two very light coats. The only time it causes a problem is when it's hammered on too thick over exposed feathered edges and the thinners content softens the feathered edge. Some paint companies even suggest using AP as a wet bed for blends, so in itself it isn't that bad, just like any 1K product, has a low solids and high thinners content.
Chris



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PostPosted: Sun Aug 25, 2019 7:52 am
Thanks^^^

I sanded on the rear bumper cracks and It looks like they eventually disappear but I worry about taking away too much material. I remembered I had another rear bumper in the shed but it was a piece I used to practice spraying on years ago so I’ll have to see what it looks like under many coats of paint. It may be a better piece.
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PostPosted: Sun Aug 25, 2019 2:24 pm
Hmmmm....when I blow that last pic. up and put filters over it I see a LOT of cracking still left down pretty deep.....If you indeed have another that's showing less deep cracking it would be better.
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PostPosted: Sun Aug 25, 2019 3:09 pm
DarrelK wrote:Hmmmm....when I blow that last pic. up and put filters over it I see a LOT of cracking still left down pretty deep.....If you indeed have another that's showing less deep cracking it would be better.


Oh yeah there’s a lot of cracking. I started sanding on the other bumper, it looks better. The problem now is that that bumper has about 20 layers of paint on it and it’s being difficult to sand. I may try a paint stripper on it.

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PostPosted: Sun Aug 25, 2019 6:04 pm
You can use strippers on them however.....first, you should try to use a bumper stripper to reduce the chances of interaction with the bumper material, second, do not take the stripper ALL the way down through the paint package. When you get within a couple of layers, stop, and neutralize. Then go back to sanding.
Stripper (no matter what the safety claims may be) can easily react and melt plastics. Here is a current one that is not too bad....
https://www.summitracing.com/parts/smm- ... 5EQAvD_BwE
I have been in the chemical stripping business for 40 year now doing metal, woods, fiberglass, and SOME plastics. Even though we are set up for it....we still prefer to sand off coatings on fiberglass and plastic. I've seen a lot of careless chemical stripping (NO, no us) melt and ruin a lot of plastic and fiberglass.
Metal, wood, fiberglass, we work it all... www.furniturephysicians.com We can restore the irreplaceable!



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PostPosted: Mon Aug 26, 2019 4:58 am
DarrelK wrote:You can use strippers on them however.....first, you should try to use a bumper stripper to reduce the chances of interaction with the bumper material, second, do not take the stripper ALL the way down through the paint package. When you get within a couple of layers, stop, and neutralize. Then go back to sanding.
Stripper (no matter what the safety claims may be) can easily react and melt plastics. Here is a current one that is not too bad....
https://www.summitracing.com/parts/smm- ... 5EQAvD_BwE
I have been in the chemical stripping business for 40 year now doing metal, woods, fiberglass, and SOME plastics. Even though we are set up for it....we still prefer to sand off coatings on fiberglass and plastic. I've seen a lot of careless chemical stripping (NO, no us) melt and ruin a lot of plastic and fiberglass.



Eugh, well I’ve already stripped it down to bare plastic in spots. Whoever had my parts car (where the bumper came from) had rattle canned over the original paint then I just scuffed it and used it as a test piece to practice spraying. There’s so many layers of paint. I never thought I’d need this bumper.

10 years ago these cars were all in the junkyard. There’s zero in my area. At this point I’d prefer to buy another bumper but it’s looking like I’m going to have make one of these work.



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PostPosted: Mon Aug 26, 2019 6:05 am
How do I neutralize the stripper?
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