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Lator_Gator 12-19-2006 03:39 PM

Urethane primer and body filler
The cab and fenders of my '51 GMC are currently in primer. It had an etching primer sprayed on first followed by Nason Urethane primer.
The parts need some minor body work; minor enough that I figured I would have the parts blasted and primed so I could do the body work over the primer and not worry about flash rusting. Well, it was supposed to be epoxy primer but I'm not going to go into that right now.(The remaining sheet metal has been epoxy primed)
My questions are:

What are my options for applying body filler?
Must I sand the effected areas down to bare metal prior to applying filler?
If I sand down to bare metal, should I use a self etching primer? Can I apply filler over that?
Can I scuff the urethane and apply filler over it?

I need some guidance. Any help would be much appreciated.


colormecrazy 12-19-2006 05:00 PM

OK lets see now? In my opinion, because I don't like etch primer at all, you should sand all of that off before spreading filler. Then spray the bare metal with epoxy before you do the filler work, it's my understanding that the epoxy under the filler actually increases the adhesion of the filler. Remember, you can apply filler over non-sanded epoxy if you do it in time (a week or more in some cases, check the Data sheet on the epoxy you choose). Then re-coat the panel with epoxy after your done with filler work, to seal exposed metal, and then prime with urethane primer. I have a 68 Mustang in the shop I've been working on this week. It was sand-blasted and had a thick coat of etch primer on it, before it came to the shop. I've been sanding it all off panel by panel, before I do the body work. The reason for this is, I personally don't trust the adhesion of the etch primer, especially when someone else put it on. Etch primer is an acid based product that must flash for a while before you prime it, and it's not uncommon for paint staff to neglect this in our go-go-go atmosphere. Acid remaining in the etch primer can effect the integrity of the primer put over it. Epoxy has the same down-side you need to let it flash a while before priming over it too. But, epoxy has adhesion qualities that are far better than acid etch. So, anyway, it's not really the easy way, and probably not what you wanted to hear, but (in my opinion) it's the way to do it right! :thumbup:

Good Luck!

Lator_Gator 12-20-2006 06:12 AM

You're right, that's not what I wanted to hear. I really hate to strip off all that primer. Is there any other alternatives that will net good results? If I REEAALLY have to remove the Etching and Urethane primer for best results and apply epoxy, then I may do that.

Bummed out in Kansas

Lator_Gator 12-20-2006 05:34 PM

Okay, let's assume that the etch and urethane primer was layed on properly. Can I apply epoxy over that allowing me to use the filler on top?
BTW, it is Nason Urethane primer.

unbearable 12-20-2006 06:21 PM

no no no ... you must switch to SPI primer or your doomed.

x711 12-20-2006 06:47 PM

Gator, If you scuff the Urethane, you might be just fine to apply the filler
but if you suspect that the primer has not got good adhesion then that
will be a weak link.

I would remove the primer to bare metal.
Apply filler "Not Bondo" I use Metal 2 Metal as my filler of choice !
Top coat this once shaped and smooth with the primer of your choice
Epoxy, or regular 2k.

If you do a search on the board here, there were some lengthy discussions
on this subject..

Try the search feature.


adtkart 12-20-2006 07:44 PM

Applying filler over urethane primer is not the way to go. It should be stripped to bare metal, prepped with 80 grit and sprayed with epoxy primer, then any filler work can be done on that. The urethane will not have the adheasion needed for supporting the filler.

The epoxy will provide moisture protection and the holding power for the filler.

It may be more work than you want, but it needs to be done. If it's worth doing, it's worth doing right, so it will last.


Lator_Gator 12-20-2006 10:05 PM

Message received

Thank you for all the advice. A few more questions: Since there are only a few spots on the back side of the cab that need some minor filler work, would I be safe in removing the primer on those areas, applying filler directly on the metal (I know alot of you suggest putting it over the epoxy primer, that's exactly why I wanted that primer to begin with) then prime over the filler?
Also, the cab was completely blasted inside and out then primed inside and out. Would you take the inside down to bare metal too? Would it really be necessary?

colormecrazy 12-20-2006 10:23 PM

If you don't need to apply filler to anything but the back of the cab, I would be agreeable with only stripping the back of the cab doing the necessary filler work. Then use 120 to block-sand the entire exterior. Then seal the whole mess with some epoxy primer and some more urethane 2K primer wet-sand and shoot. If you are confident in your existing primer it should be an OK substrate for the build of epoxy and more primer, but as stated not really the necessary foundation for filler. As for the interior the existing will probably be OK, because it will not receive the abuse of the exterior.

adtkart 12-21-2006 03:56 AM

What he said. ^^^

gtcway 12-21-2006 08:24 AM


baddbob 12-21-2006 01:16 PM


Originally Posted by unbearable
no no no ... you must switch to SPI primer or your doomed.

Oh, so you've tried SPI's primers?

baddbob 12-21-2006 01:26 PM


Originally Posted by Lator_Gator
Okay, let's assume that the etch and urethane primer was layed on properly. Can I apply epoxy over that allowing me to use the filler on top?
BTW, it is Nason Urethane primer.

Yeah, you can. The only drawback is during the sanding process you'll likely cut through the primer and if the etch primer is exposed there's no putting more filler on that area. Etch primer is forever soluable and fillers shouldn't come in contact with it. Some epoxies aren't compatible with self etch primers.

Lator_Gator 12-21-2006 09:22 PM


You mentioned the etch primer being forever soluable so I'm wondering how I strip that off with no residual effects if I were to strip the back of the cab?
Perhaps I should just scuff the whole exterior and shoot epoxy over it all, then do the filler. I'm only looking at minor dents that should hammer & dolly out pretty well.
Here is what I'm thinking of as a plan of attack. Please correct me where I've gone wrong:

1. Hammer & dolly out the dents/dings as best I can. (Touch up chipped urethane primer with same primer.)
2. Scuff the exterior with 120 grit.
3. Spray on the epoxy primer. (How many coats?)
4. Apply filler where necessary. (How long do I wait after applying epoxy? Scuff epoxy first?)
5. After sanding down the filler (what grit for final work?), apply a filler primer(a couple coats?).
6. Block sand everything.(What grit?)
7. Touch up as necessary.(Respray everything?)
8. Apply final primer.
9. Apply sealer.
10. Ready for color.

Thanks again to all for your willingness to help. I'm sure alot of you can do this kind of stuff in your sleep. For those of us that can't this info. is invaluable.


MARTINSR 12-22-2006 03:05 PM

What are your expectations? Really, there is no reason to remove that etch and urethane primer. MOST shops around the country wouldn't do it any different. For that matter many (if not most) wouldn't even use the etch and apply the urethane over the bare metal.

If you are after the most wiz bang job you can get, strip it and apply epoxy. If you are simply after a nice job that 90% of the cars you see repaired or restored have, work with what you have.

How much filler work are you going to need?


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