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Corn Cob and Walnut Media on Thermoplastic Panels Info Please

#1
So I have had this job on the back burner for a few months for a few reasons. Pretty much a sand and shoot(no rust repair, only minor dings, etc). Its a dream job, except one thing.
It has a factory body kit with some sort of thermoplastic bumper covers, side skirts and wheel arch/flares. And as is typical with an 80s car, the factory applied paint/primers are hard as a rock, and the bumpers appear to have been resprayed once. Both the original paint and the respray I assume is some kind of single stage enamel.

Anyhow, so the part of this I have been dreading is stripping this stuff. I have done a few of these in the past, and using 80 grit 3M gold on a orbital takes literally days. Because the paint/primer typically has cracks all the way to the plastic, you just about have to use 80 grit til you hit the plastic, and then it wants to get "fuzzy" so then you have to go over it with 220 for another several hours.

Between the time and money spent on paper(plus all the primer and subsequent sanding), I have been trying to find out about stripping using walnut shells and corn cob media. As a reference, I am familiar with using crushed glass. I routinely use 40/80 mesh through my ALC pressure blaster. But these vegetable and nut products is something new, and doing it on plastic is also new.

So, I guess big questions are walnut or corn cob(or other)? What mesh/grits? As long as I am not a moron, how easy is it to create a bunch of low spots? Any other tips?

Thanks!
 
#2
I'm going to tag along with you, since I could use an answer to this, too. Most of the time we simply replace bumpers that have bad paint on them, it's quicker, cheaper, and better. But there's always those ones that can't be obtained, and then the pain begins.
 

Barry

Paint Fanatic
Staff member
#3
Walnut or glass should be fine, I hear more and more the last 4-5 years of shops using sand carefully to strip bumpers.
Happens a lot with aftermarket bumpers that they don't test the coating it comes with than wrinkles to all get out.
 
#4
material means little. the MOH rating is the key. i use starblast , acrylic and corn cob. i run less pressure than most . 35-40 lbs and very little media . personally i would run backwards from blasting plastic :)
 

Chris_Hamilton

Trying to be the best me, I can be
#5
Watch your pressure as Shine says. Too much pressure you will embed whatever blasting material you use into the plastic. Again like Shine said 35-45 lbs. Test a small area or something similar first.
 
#6
Thanks,
I am picking up some sacks today from the supplier. Apparently they dont stock the walnut here, so I ordered 5 sacks of corn cob, and another 5 sacks of a soda blast they suggested as well.
I have a handful of scrap bumper covers around and will try playing with it this weekend.

I'm not exactly running towards blasting these plastic parts Shine :) But, between the side skirts, wheel arches, and the bumper covers and all the little pockets, the last couple sets I did using sand paper to strip took me at least 40 hours. If i was sanding a modern base clear that would be one thing, but man, that old acrylic stuff sucks. So figure I will give it a whirl.
 
#8
Set up for it? I'm not sure Chris. Below is a shot of my blaster, and its running a 1/8" tip.
I thought I had a 3/16 tip as well(but it may have been 3/32) but dont see it around right now.

Also below is a pic of a real bad spot on the situation I have to deal with. This is the only spot where it's peeled but it's all gotta come off.


20180323_182448.jpg
20180323_182503.jpg 20180323_183033.jpg
 
#10
Ah cool thanks. I guess on some models they have to add a way to drain the media from the tank and stuff. Mine has a plug on the bottom I can remove.

Well gave it a shot on a scrap bumper today. The corn cob was brutal. It was a large grit and wanted to plug my 1/8" tip, and there was a fine line between paint removal and this weird smearing or almost like pushing of the plastic.
Drained the corn cob out and went to soda. That was a winner!!!! It leaves a very fine texture that the 2K will fill easily I believe, and worked fast.
I did this area about the size of a license plate in about 45 seconds!!

20180324_141407.jpg
 
#11
I don't know that procedure for neutralizing soda residue on plastic. I do know that we won't use soda on metal because SPI has not recommended it in the past.
 
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#12
Yep. I do need to research the neutralization process(I think consists of washing it), but i have time for that.
Probably start working on the car in a week or so. Just wanted to test the process so now I can sleep a little easier.
It's pretty reasonable too. It is like 22$ for a 50lb sack.
 
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