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Old 09-13-2006, 08:20 PM
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question on primer adhesion

I stripped the body (52 chev 2dr) to bare metal and metal prepped and epoxy primed. I now find that when I am doing the bondo (using 40 grit) that the epoxy primer does not appear to be adhering to the metal that well. At least I don't think it is, maybe I am wrong. On the places where the grit has scratched the primer (off to the side of the bondo) to the bare metal the primer that is left in-between the scratches can easily be scratched off with a finger nail. Is that normal?? Or am I going to have to strip the primer off and try again. Maybe I put the primer on too heavy?? I used DuPont M-3 metal prep.

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Old 09-13-2006, 08:46 PM
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What grit did you use to prep the metal for epoxy? I use 80 then wax and grease remover and then run over it with red scotch brite and blow it off at the same time to get rid of any cloth hairs. Then I shoot two wet coats of epoxy. It shouldn't come off unless you are really diggin at it or unless its not cured out. Is it a descent brand of epoxy???
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Old 09-13-2006, 08:50 PM
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Epoxy needs a clean and textured surface for good adhesion, and temperatures 60F + for it to cure at a normal rate.

How long ago did you spray the primer? What temperatures has the car been seeing? What grit sandpaper was used on the bare metal before the epoxy went on? What was used to clean the metal after the metal prep? Is the metal prep suggested for use under the epoxy?

I really don't believe in the need for any metal prep, I've seen adhesion problems with the stuff many times in years back.
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Old 09-13-2006, 08:52 PM
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Epoxy

Try some laquer thinner on a rag on non sanded area. If comes off easily probably had wrong activator or did not stir the 5 min. and let activate for 30 min. Also if sanded to fine ,it will not adhere.Also how many coats , did you let it flash to long , if did solvent dried and 2 coat did not link. let it set for a day and retest.
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Old 09-13-2006, 09:37 PM
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Well, the body was stripped using sandblasting around all the hard to do areas like window openings, belt lines etc. the rest was done with 100 grit orbital sander on just circular motion and then 120 grit on orbital. I then metal prepped it as per the instructions. It was primered a few days later in fairly hot weather. Used two coats of primer and stirred well with the 20 minute activation time as recommended by the product. I guess I need to know if the primer should come off that easy. The metal prep product says to apply paint without any further preparation. Does not say if it is compatible with epoxy. Just to be on the safe side I may just remove what is left and start over. Can't do much about what is already covered by bondo. I use metal prep because I quite often don't get to prime right away and don't want bare metal to rust. There is usually spots to repair prior to priming.
Thanks for the input so far.
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Old 09-13-2006, 09:43 PM
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Bob will be able to answer this a lot better then myself as I have never used or am familiar with this metal prep. Personally I wouldn't be using it. Cheers!!
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Old 09-13-2006, 10:45 PM
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The primer should be removed. Two clues to the problem are the metal prep and the high temp. Was the surface of the metal hotter than "room temp"? If it was, if you had the thing out side prior to shooting and the metal was hot, all bets are off on adhesion. With the metal prep, I am with the other guys, don't like it. If you don't follow it right to the tee you can really screw yourself. It will leave a powdery film on the surface, you might as well prime over dust.


Brian
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Old 09-14-2006, 05:22 AM
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I don't know what brand of epoxy you used, so don't know what they recommend. SPI reommends sanding with 80 grit or courser for bare metal, when spraying epoxy. Sanding with 120 will almost polish it, specially if you don't constantly have a new disk on it.

If you put bondo over epoxy that isn't adhearing good, the bondo won't stay either, and should come off. If you have bondo work to do, and the epoxy is applied properly, you can apply the bondo over it.

Aaron
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Old 09-14-2006, 06:23 AM
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[QUOTE=randykaty]Try some laquer thinner on a rag on non sanded area. If comes off easily probably had wrong activator QUOTE]

Or it was PPG DPLF .

Vince
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Old 09-14-2006, 06:33 AM
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Everyone has the right Ideas but since were coming up on cool weather I would like to add a few things to prevent these types of problems.

First you did not say when you applied body filler over the epoxy and how soon you sanded it. Its not unusual to scrape epoxy off the next day with a scraper as it does take a few days to get really good adhesion.

I don't know the acid number you used from Dupont but did you neutralize it? Does it need neutralizing? Did you dilute it as stated on the tech sheet?
A dried acid film will kill adhesion to any product.

With cooler weather coming this is a reminder as this has been stated many time before.
Before you spray epoxy make sure the metal temp is at least 60 degrees and remember metal temp is 8-10 degrees cooler than the outside air.
Epoxy does not cure like primers and clears if the metal temp is in the 50 degree range the curing process could stop and the bottom line is the epoxy will never cure a 100% and only fix is taking back off.

If need be rent a heater (salamanders are cheap to rent) Preheat the car for an hour and after spraying the epoxy keep the heat on for at least four hours or don't spray it at all.
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Old 09-14-2006, 08:16 AM
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thanks guys, I am going to remove it and start over as I don't feel comfortable with it's adhesion.
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