I'm stripping the underside of my decklid for primer and while it is mostly rust free, there are a few small pinholes that popped through near the bottom corners. I've used fiberglass filler on pinholes like this in the past, but when I've done that, I used a body hammer to knock the surrounding metal inward to give the filler something to grab onto. In this case, I can't really do that because the outer skin is right up against panel with the pinholes in it. I'm wondering if I could use something like JB weld to fill them? Anyone use it under paint? I never have and it sounds unconventional but I can't think of any reason why it wouldn't work. It is an epoxy with a high bond strength to steel, sands nicely, and cures hard. I think it would almost be similar to all-metal without the aluminum shavings in it.

I don't see why not, GO FOR IT. one thing with JB is its brittle and won't flex much BUT in your case not a problem.
Jay D.

I did it and it worked well.

It was an old Corolla that I just wanted road legal. Around the top of the pillar next to the windscreen there was a rust hole about 10mm/ 1/4" round so I wire brushed it, cleaned it with solvent then JB Weld. Ordinary primer then single stage white paint.

I expected rust to come through at a later stage but it never did.

Obviously you know your JB Weld, but the only recommendation I can give is have it slightly below the surface, then use ordinary filler to level it, as it's VERY hard work to sand.

Bill911 wrote:I expected rust to come through at a later stage but it never did.

It will. A matter of when, rather than if.

Bill911 wrote:use ordinary filler to level it

If you must just fill the holes with something other than metal (either by patching or welding), then use filler, not JB Weld.

NFT5 wrote:

Bill911 wrote:I expected rust to come through at a later stage but it never did.

It will. A matter of when, rather than if.

Bill911 wrote:use ordinary filler to level it

If you must just fill the holes with something other than metal (either by patching or welding), then use filler, not JB Weld.

Can you provide some reasoning why? I could obviously try to weld them closed, but there is a very real possibility that I would just blow right through and make them bigger. As stated, fiberglass filler probably isn't ideal in this situation as it doesn't have a lot to grab onto.

welding would be a mess, it rusty under the holes ( rusted from the inside out) and paper thin. you could end up with a 2" hole before you know it. if there is an access hole under the door panel try and get down there with something and scrape it as clean as you can. then run a wire up the drain hole, do this with all of them. you want the water to drain out. when you get done with the scraping blow it out with an air hose. then after you use the JB weld or what ever you decide on, use a small brush and smear some paint of some kind on the inside, making sure the drain hole is clear. if its clean i like to paint the complete bottom inside. Just a tip here, but before you fill take something and indent the holes a little, this gives the filler a little more to bond to and you won't sand it off. be careful as its going to be thin there. i have a small ball peen hammer i use. i use the round end and hold it over the hole and hit the hammer face with another hammer lightly. or a bolt ground to a sallow point works.
Jay D.

JoeDirt wrote:Can you provide some reasoning why? I could obviously try to weld them closed, but there is a very real possibility that I would just blow right through and make them bigger. As stated, fiberglass filler probably isn't ideal in this situation as it doesn't have a lot to grab onto.

Sure. JB Weld is multipurpose and an excellent product, especially for patching difficult plastics and in situations where leaks need to be patched. It has lots of uses, but isn't specifically made to patch bodywork. Filler, whether the lightweight variety or fibreglass reinforced, is.

I can't and won't comment on your welding ability but I'd be pretty sure I could MIG up those holes. Settings are important, as is skill level and not overheating the edge. Try practicing building up the weld pool over good metal and then pushing it over to gradually fill the hole.

As mentioned by Badsix, JB weld is difficult to sand and you certainly won't get the nice feathered edge that you need so you'd need to use filler anyway. Why not use the right stuff in the first place?

As for having something to hang on to, note the suggestions to create a small dent around each hole. Scuff with some coarse paper and it will hang on forever.

I didn't want to do a correct job, I just wanted the car road legal. The area was pitted which suggested if I welded it I'd just get a hole burnt through it. And it was near the glass so I'd need to do a glass out, maybe new rubber around the glass, and so on. The car was worth less than $1,000.

So,, I got the 1" area very clean. Stainless steel wirebrushed all signs of rust out of the steel. Gave it a quick sand blast with my mini (about the size of a spraygun) blaster, a scrub with phosphiric acid, washed the acid off before it dried. Go it completely dry with a heat gun. Put JB Weld in the actual hole. PPG epoxy primer, ordinary sandable filler, primer, sanding, primer, sanding, white single stage. And a polish of the area to hide what I'd been doing.

I live right at the beach, so plenty of salt air, and no rust showed up for the next two years of me owning the car.

NFT5 wrote:

JoeDirt wrote:Can you provide some reasoning why? I could obviously try to weld them closed, but there is a very real possibility that I would just blow right through and make them bigger. As stated, fiberglass filler probably isn't ideal in this situation as it doesn't have a lot to grab onto.

Sure. JB Weld is multipurpose and an excellent product, especially for patching difficult plastics and in situations where leaks need to be patched. It has lots of uses, but isn't specifically made to patch bodywork. Filler, whether the lightweight variety or fibreglass reinforced, is.

I can't and won't comment on your welding ability but I'd be pretty sure I could MIG up those holes. Settings are important, as is skill level and not overheating the edge. Try practicing building up the weld pool over good metal and then pushing it over to gradually fill the hole.

As mentioned by Badsix, JB weld is difficult to sand and you certainly won't get the nice feathered edge that you need so you'd need to use filler anyway. Why not use the right stuff in the first place?

As for having something to hang on to, note the suggestions to create a small dent around each hole. Scuff with some coarse paper and it will hang on forever.

Good points. I'm not saying you're wrong, I was just curious about your reasoning. I can stick metal together but I'm not a professional welder. Sometimes my welds come out great, other times, not so much. I have MIG'd holes shut lots of times but it can be tough when the metal is thin.

I've used U-Pol Fibral Lite for pinholes and I really like it. Its a great product. But it is hard to peen in the metal around the pinholes because the trunk skin is right behind the inner structure with the pinholes in it.

JoeDirt wrote: But it is hard to peen in the metal around the pinholes because the trunk skin is right behind the inner structure with the pinholes in it.

A light tap with a small ball pein hammer will do it. Only need to drop the surface 0.5mm. I use a Dremel sometimes when I want to only work in small areas - the sanding discs are only about 15mm diameter, or there are other accessories like miniature drum sanders. There are also drill bit accessories that will work.