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Anyway to field test epoxy adhesion?

#1
I laid down 3 coats of epoxy primer in the rear windshield and trunk gutter on my project car, 3.5 days ago.

Years ago i sandblasted it and put down POR15, which held up great. I used a really stiff wire wheel at low RPM to peel that stuff out for Epoxy.
Last night as i was drifting off, i woke up in a cold sweat... i cant remember if i went over those areas with sandpaper after i wheeled them. I probably did, i just cant remember doing it. I sanded half an acre of other metal on the project and can remember that. So im worried i missed that step, and these areas tend to rust if not perfect. I do recall thinking that the wire wheel i used left some deep scratches in the metal, and didnt really polish it... but i had fulled intended to aand anyway.

Its hard to get a razor blade in thier to do that test. I put some packing tape on it and pulled it twice in the same spot, nothing came up. Is there another way to test it? If not i supposed ill remove it and do it again, but would rather not.

Thanks!
 
#2
I only know of the destructive testing for our military work, cutting thru the paint with a razor, making a checkerboard with around 1/4 inch squares, then putting the tape on it to pull up the paint left on the chip.

I am pretty sure the scratches left by the brush were around 80 grit. I mean, the 80 with the DA is a nice random pattern, and wire wheel usually go in the same direction, but you are creating craters for the epoxy to wrap onto.
 

Barry

Paint Fanatic
Staff member
#3
After 3 days take a flat tip screwdriver and get it under epoxy if epoxy is going to stick in 3 days all you will minor chip.
I have always warned people my epoxy will not stick to that suff evenif sanded with 24 grit grinder.

Even if sticks now that does not mean it will a month or 6 months from now..
 
#4
I have found that the epoxy really likes 80# scratches, but what it likes even more is clean metal. If you know it was clean bare metal, I wouldn't worry about it.
 
#5
After 3 days take a flat tip screwdriver and get it under epoxy if epoxy is going to stick in 3 days all you will minor chip.
I have always warned people my epoxy will not stick to that suff evenif sanded with 24 grit grinder.

Even if sticks now that does not mean it will a month or 6 months from now..
Are you referring to the POR? If so, thats all gone. It was bare clean metal, i just cant remember if i went over jt with 80 after wire wheeling it.
 
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Barry

Paint Fanatic
Staff member
#6
Sorry thought the por was still on there.
Your fine, temp good if you mixed it right, no need to test.
 

Barry

Paint Fanatic
Staff member
#8
Here is a test I use when a shop calls and says not sure my guys activated the epoxy right.

In 24 hours take a razor scraper to the middle of a flat surface you may scrap 3 to 6 inches off the metal.

Next day do same but not near the first test you may scrap an inch or so off

Next day you may chip the epoxy and may see some metal but you usually won't.
Now I can say to shop owner the painter mixed the epoxy perfectly.
 
#10
Duct tape test is useless with epoxy.
20% adhesion will beat the tape test.
Scotch 250 is the specified tape for destructive testing. its about 70 a roll. Super strong rubber adhesive is one difference, other than a shiny backing that does not tear like cheaper masking tape. Usually what you see are the little chips of what you cut in the opposite pattern and the squares stay on the part if its good. But its destructive, so you are reworking where you are checking anyway.

I dont know what the real end cure of epoxy is with the chemical cure, we have heard let is set for a few days, to a week so it just gets harder and harder over time. I am not sure if a failure day 1 would still be a failure day 7.
 

Barry

Paint Fanatic
Staff member
#11
Epoxy has its weakest adhesion in the first 24 hours; I know gorilla duct tape won't touch it.
Never heard of 270, but be interesting to see if it did anything but if you have to crosshatch you're destroying the coating, so why not take it off and do it right? The test is a waste of time it sounds like to me.
 

Barry

Paint Fanatic
Staff member
#12
Failure day??
Full cure on the spi epoxy is about 90 days
At that point very hard to sandblast off and if an outside skin, you will not blast off without warping the panel.
Now, acid films depending on the strength
Of the acid, you can lose adhesion to point just slapping panel will knock off, or it won't feather or maybe beautiful until you get a stone chip and it starts peeling.
 
#13
Epoxy has its weakest adhesion in the first 24 hours; I know gorilla duct tape won't touch it.
Never heard of 270, but be interesting to see if it did anything but if you have to crosshatch you're destroying the coating, so why not take it off and do it right? The test is a waste of time it sounds like to me.
We are required to send chips of the base metal, prepared the same way the parts were, apply the conversion coating, prime, tape off an area, topcoat, cure, then do the cross hatch on the painted area to prove adhesion. With Raytheon jobs, we also have to measure both layers of coatings, which is why the epoxy is masked off.

If the chip survives, they expect the parts to survive. All it was was the peanut gallery giving an example on how to check adhesion of epoxy in a different universe.

In his case, you would destruct a panel, but your mind might be at ease if you test the part you are most worried about and it survives, you are only respraying that little area.
 
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Barry

Paint Fanatic
Staff member
#14
Lol, I feel for you as I can see a lot of things wrong with this test.

Reminds me of years ago, sitting at a table with a vp from a major.
I said, why do you require your sales people to have a ba instead of paint experience like its been for years. Because all the paperwork they need to do each week.

Oh got it, they create work to keep ten middle managers busy all week, trying to be impotant (did I spell this wrong? Nope)
 
#15
I'll take hands on experience any day of the week and twice on Sunday over a schooled book worm that is now considered "THE" goto professional who knows best.


Lol, I feel for you as I can see a lot of things wrong with this test.

Reminds me of years ago, sitting at a table with a vp from a major.
I said, why do you require your sales people to have a ba instead of paint experience like its been for years. Because all the paperwork they need to do each week.

Oh got it, they create work to keep ten middle managers busy all week, trying to be impotant (did I spell this wrong? Nope)
 
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