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starquest 01-31-2006 05:32 AM

Prepping urethane bumpers and fiberglass parts?
I'm at the point in my restoration project to start prepping the urethane (I believe they are urethane) bumpers and other plastic parts of the vehicle.

This is my process for the STEEL parts (as recommended from you guys on these message forums):

1. strip with aircraft stripper
2. sand with 80 grit
3. clean/tack
4. SPI epoxy
5. filler
6. re-epoxy
7. SPI 2K
8. block

So what is the process for the plastic parts? Can I continue using the aircraft stripper? What grit do I prep the parts with for primer?

The parts left to prep and put in primer are the urethane bumpers, urethane 3 piece rear spoiler, urethane chin spoiler, plastic grill, fiberglass hatch, and plastic stock hood scoop.


BondoKing 01-31-2006 07:42 AM

Stay away from the aircraft stripper and just use 80 grit... put 2 good coats of epoxy, let set over night, then 2k next day and block...

BTW on your metal, I hope you cleaned the metal really good before you started running the DA with the chemical stripper.. If not chances of you having problems are almost gaurenteed... You need to remove such things before sanding and grinding it in the metal or plastic parts for those who use it...

I used it years ago, but I no longer do, too many issues can come from it vs just sanding it down

x711 01-31-2006 09:13 AM

Dont use Aircraft stripper on those Urethane/Plastic parts or you will be in a world of hurt.

To sand them smooth your better off starting with a higher grit sandpaper.
While a lower grit such as 60 / 80 etc will work you risk gouging the surface.
I use 180 to 220 to start, then work up from there typically to 320.

For priming the parts I use this product below, I am sure others will also work.
It has a flex agent built in. You can use this for the whole car so there is no
need to have different types of primer for a particular job. Its basically 3 products in 1 depending on how its reduced.

================================================== ========
XTREME #5425 is formulated with a hybrid of epoxy and acrylic polymers, which provide excellent adhesion, good corrosion
resistance, productive dry times, and ease of sanding.

Its made by 5 star autobody products.
================================================== ========

I made the mistake of using Aircraft Stripper on a Urethane based bumper cover one time, It sure did get rid of the paint, but it also started to melt
the bumper too !

Thanks.... X

starquest 01-31-2006 09:30 AM

Thanks for the replies.

Yes all surfaces were thoroughly cleaned after stripping and prior to DAing. It was sure a time consuming process but it beat the other alternatives; lots of DA/hand sanding or sandblasting (its jap steel so there is a lot of chances of warping).

Thanks for the steps. I guess I have a lot of work ahead of me with the DA! lol, that is ok though.

But one of you suggested just using the SPI epoxy/2K while the other suggested a different brand combined with a flex additive. Which route should I follow? Is there a flex additive for my SPI epoxy and 2K, or don't I need it?


BondoKing 01-31-2006 10:51 AM

Kevin flex additive is a marketing hype... it is designed to aid in putting the parts back on, allowing for more flexibilty... With todays resins ( spi for one) that is not needed at all... and to top it all off, that flex additive will be gone out of the bumper in around 30 days in 70-75 degree weather... Dont waste your money on that one bro

jcclark 01-31-2006 10:59 AM

This is a quote from another forum on flex additives.
I think we've been misinformed on here.
The mfg's says it is permanent and keeps the paint softer.

"I used to believe the same thing, because sooooooo many people will tell you that flex will 'wear out' of the paint and it's only temporary. Many times they'll add something like 'just phone the manufacturers and they'll tell you the same'. So a while ago I did just that, I phoned dupont and asked them if flex was only temporary. THe guy sounded a bit like he had never even heard of that before, and of course it's not temporary. It lasts, it's permenant, the paint won't return to "original hardness". Just phone the manufacturers and they'll tell you the same!"

jcclark 01-31-2006 11:13 AM

I do a lot of bumpers and I usually use stripper, I haven't had one melt
yet but I'm careful as I go to watch the progress.
Some of todays bumpers have so many little openings and grill work that
I don't see how it would be possible to sand all the paint away.
I have had some "fuzz-up" from sanding when dry sanding, so I avoid that.
Usually after stripping I wet sand the whole thing with 400 just to remove
all the residue from the stripper.
Some people don't prime bumpers before painting, they just apply a coat
of "Bulldog" adhesion promoter and paint. I'll do that for small sections but
for a complete bumper I still like to prime with a good 2K.
With the "el-cheapo" bumpers I usually paint I need the 2k for smoothing
out the imperfections that they're known for.

BarryK 01-31-2006 12:07 PM

The older flex agents did lose effectiveness over time.

If the flex agent uses "Polyglycidyl Ether Polymer"
(Excuse spelling as I don't have access where I'm at.)

This additive is a plasticizer and will remain flexible, only problem is its not required to be listed on MSDS or Label so how you will know, I don't know.

jcclark 01-31-2006 01:00 PM

I know that I don't know, you know? :pimp:

I haven't used the flex agent in a long time and the bumpers
I've done have held up just fine.
My daughter has tested the flexibility out on steel panels for me,
(she smashes something regularly) and the paint has stayed on
amazingly well. So I'll keep painting without it.

x711 01-31-2006 02:59 PM


The 5425 primer has the flex additive built in, "You do not have to add it"

I mentioned this because you do not want to use primer on a flexable part
such as a bumper cover etc that could possibly chip away or crack if the
part was flexed etc. Keep in mind, Paint goes over the primer. If the primer
cracks or breaks away.. So will your paint !

Thanks.... X

roger1 01-31-2006 03:35 PM

Captain Lee's Auto Spra-Strip is what is recommended for fiberglass Corvettes.
I just used it and it dissolves the paint but not the primer. And the Corvette I just did was a '69 and it still had the factory primer on it. So it didn't even get to the fiberglass. I sanded the primer off afterwards. That was still a lot easier than sanding off the 2 layers of paint that were on top.

It is not supposed to eat into fiberglass at all.
Don't know about using it on urethane or plastic parts though. But if it doesn't go thru the primer, it should be OK on that too.


adtkart 01-31-2006 07:56 PM

I have a mixing cup in my garage that had SPI epoxy mixed in it. I have tried to get the stuff out. I have definately tested the flexability of the stuff there. I sure don't think it needs a flex additive in it.

I would go with the SPI epoxy on a bumper cover at any time, infact I am in the process of that right now.


baddbob 01-31-2006 09:16 PM

Aaron, Isn't that SPI epoxy some amazing stuff? I did some flexibility tests on steel panels with bending them 180 degrees and couldn't get the stuff to fracture, this was after a four week cure! Bob

adtkart 02-01-2006 03:56 AM

Bob... Since I work in a shop where we replace alot of bumper covers, I am thinking about bringing some small bumper pieces home from work to test. That stuff really sticks to everything, so I want to do some tests for the flexability, and really test the adhesion on them too.


jcclark 02-01-2006 07:44 AM

The thing to remember about flexibility, it's a time issue.
All things will harden eventually, it may take 100 yrs, but eventually.
I have had some clears stay flexible for a couple of years then
get brittle, so how much time is enough for a test?
And how much is acceptable?
I usually guarantee my paint for three years. That's what I shoot for.
I have redone some other body shops work that took that long to show up.
Especially blends, where the clear started to show differently.
My own "test" paint job with "Lazer-Dry" looked perfect for 3 yrs,
then went bad quickly.
That's why it's important to remember that just because the paint looks
perfect after being done or a couple of months doesn't mean
it's a really high quality paint job. Time has to pass.
There have been girls and guys on here giving advice about their own high
quality paint jobs and only started painting a year ago. That's not enough.
The real test is time, and for me the three year period seems to reveal
what really has been done.
So if you want to check flexibility or adhesion, I want to see it on
something at least a couple yrs old, then we'll know. :pimp:

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