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Primer + Bondo


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I have read some of the info stating that using either a self etching or epoxy primer under the Bondo really extends the life of a Bondo patch. Unfortunately I read this AFTER I had done the patch and all the sanding, so there is no primer underneath my Bondo. Is it really worth it to grind the Bondo out and redo? Since I don't have access to a gun, I am forced to use whatever primer comes in a spray can. Any recommendations on brands that will work well under the Bondo? Any tricks or proven methods are appreciated. Thanks, Victor.

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The main problem with NOT using an etching primer below the bondo is that if there is ANY way for moisture to creep in between the bondo and the metal you can be assured that it WILL eventually rust and cause the bondo to pop off.

The kinds of problems that would indicate that are: holes in the metal for "fingers" of bondo to hold tighter. If the bondo is on a seam or crack in the metal that hasn't been thoroughly welded / brazed closed. If the bondo comes close to the edge of the metal (such as panel edges) where the paint can get real thin and therefore chip. And lastly, Flex Points on the car body. These tend to crack both the bondo and the paint and therefore allow moisture to creep in.

Now, even if you were to etch primer or epoxy primer, or even zinc plate the metal there are NO guarantees regarding rust-proofing metal. At best you will be given promises with conditions. Heck, even POR which many of us have used and like has these provitions in their warranty paperwork.

So having said all that.

If your choice were to use a rattle can of primer before you put on the bondo, I wouldn't grind off the bondo to primer. Reason for this is that you can't really get a good even coat with a rattle can. Yes there are guys out there that are very good, but I'll still take any one of them on with my primer gun. Secondly, I can't think of a single brand of epoxy primer in a rattle can, and the same goes for etching primer.

What I would do, is make sure you smooth off all your bondo so that you can lay a good coat of Fill Primer. Wet sand that down so that it finishes the panel smooth. Then using a Primer SEALER give it one last coat. You might also be able to get NON-sanding sealer and then top off with your paint.

Can I guarantee that you'll never get rust in that panel? Nope. But then again, I couldn't even if you had used Etch Primer.

Years ago, (Mid 80's) we didn't use etching primer as a matter of course. We used good old Ditzler Red Oxide or Gray Oxide and made sure we had a good clean surface to apply it to.

Bondo was applied to bare metal with deep scratches left in it by 16 or 24 grit grind pads, and the stuff would last for YEARS on vehicles used continuously in winter weather and even with road salt. Did some of them eventually rust? Yes. But I'll be honest that the rust could be from regular use and not because of prior repairs.

So, hope this helps.


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The repair was at the right rear corner next to the tailight, two shallow(<1/8in), narrow(1/4-1/2in) 4 inch grooves caused by the right bumper end being pushed against the body from something hitting the right bumper guard. So the bondo does wrap around the rear corner. At least it will be protected by the bumper(unless someone kisses the bumper again). I guess I'll try the fill primer and sealer primer as you described and leave it at that. I know it won't be good as new, especially with my primitive sanding skills, but the bumper should hide most of my Bondo art. The more difficult task ahead will be adjusting the shape of the right rear bumper mounting flange so that the bumper will have the proper clearance from the body. If I could see an undamaged one I could get a better idea of how I need to bend it. Anyway, thanks alot for the advice. Victor.

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