I’m but welding some patch panels and noticed that in a couple of spots I didn’t get good penetration. Is it acceptable to weld on the backside of the weld to get the penetration?
I'm guessing that you used a mig welder with too low of amp setting or went too fast. You have to remember that when you weld on a panel, it shrinks. If you weld on the back side, it will shrink some more. Trying to hammer on dolly to stretch the metal back will probably result in cracking because of the mig weld being so hard. I would grind the spots that have no penetration and hammer and dolly that area until it is back to normal and then reweld those areas only. Using a stitch weld to keep the heat down.
Is it even necessary? I mean how much stress is on a patch panel? I'm asking because I just welded a couple of patch panels on a pair of fenders and after they came back from the sandblaster there are a couple of places where the light is leaking through. I'm seriously thinking about a coat of Bondo (or equivalent), primer on both sides and paint. Yes, I used a MIG welder and my trusty grinder. I'm obviously not in the class where after welding and metal finishing the panel can be chrome plated on both sides. Some filler is fine by me and I can't see the back of the panel anyhow.
If you are considering not welding the holes it is worth considering that many fillers are porous so if you fill over a hole then moisture will penetrate from the other side and eventually (quite quickly depending on where you live) the filler will swell and move away from the panel. So you need to seal the hole (before filler is better) but can be after if not too long after. The other thing you have not said is if the patch panel is in a structural load application eg near a door hinge or A pillar. If it is then fully weld is the way to go but if not then its not so important if you can seal the backside of the hole to moisture.
Welllllll...here's what my painter told me. IF you can get to the back side of the panel you can cover over it with fiberglass. That will seal it up from moisture. Then bondo over front as usual. The issue comes when you CAN'T seal the back and moisture gets to the porus bondo. I understand they make water proof bondo now!! So far....three years have gone by and no bubbles in the paint. The car is NOT garage kept. jus say'in 6sally6 PS...If its EZ to get to....just run a bead on the back of butt weld to fill in holes.
Clean , Clean metal is the answer to good welds . It needs to be cleaned after welding also if filler is to be added .
greetings 49Caddy ( and others!) I have often read of and must agree that mig welds are hard . I have also read of guy TIG Welding using MIG wire for filler metal and am curious if this yields just as hard a Weld. ( I'm thinking perhaps the TIG Weld uses more " parent" material and less filler metal perhaps?)
Never tried that mix; but I think there are different alloys used to match the welding process and incorrect filler may not yield an optimum weld. I tried using some TIG rod for gas welding the roof on my friends car; welded weird so I quit. Brought some gas welding rod from home next day; welded normally. I also tried some MIG wire for filler when gas welding; didn't seem to weld normally either.
TIG and Gas welding slowly heat the panel, and then it takes a lot longer to cool, which is an annealing process. MIG heats the metal up very quickly, and cools much faster, which is a hardening process. If you do use a torch to re-melt, or just bring a MIG weld to cherry red and allow it to cool, it will become soft, and easy to hammer and grind. Still the same wire, but a slow heating and cooling cycle anneals the weld bead. You would want to grind the mig weld almost flat before heating the metal and hammering.
Tig welding but using mig wire as the filler results in an easy to work weld, so it's the welding process and not the filler wire that makes the difference. 1946caddy has explained why there is a difference. The beauty of using mig wire is simply that it is very thin and requires little if any grinding after it's been plannished. 0.6mm mig wire is considerably thinner than say 0.035", which is roughly 0.9mm. Chris
If its solid and no holes, you're good. You can fuse the panel, no filler wire, and it will be plenty strong enough. Only add enough filler to make it solid. I'm talking TIG or gas welding here. Forget what you know about wanting to see full penetration like you would on something structural, its just not necessary. I know, I didnt believe it either, but use some scraps and prove it to yourself. Just fuse it, hammer and dolly it, planish hammer it smooth, whatever , and then try to break it, by bending it. It'll be plenty strong. Good Luck.
Thanks for all the tips. Some of the patch panels are structural, A and B pillars. Also on the chop. Not many spots but I was just wondering if this was an acceptable weld. This is a 37 Ford humpback.
Nope, rods looked the same other than the alloy number pressed on the end, never cleaned the coating off gas rods in 50+ years, don't feel that was an issue.
I tig sheet metal using E70S6. I mount a small spool of .045 on the wall, cut off 12" lengths, run them through a straightener and use them as is. Makes a nice bead that's easy to planish.
One more opinion for you ! Whenever I try to shortcut something I always regret it later. So after many years I learned that putting in the extra effort to do something right the first time always pays off later....meaning no problems later. I think since you have a Mig and not a Tig, you should concentrate on how to make the Mig do the job. I'd start with using .023 wire which will work best if you put a .023 liner in your welder. The wire is easy to get crimped if you try to use the larger liner. I would also do stitch welding like 46 Caddy mentioned. I have just modified a dashboard where I used .030 wire and stitch welding. It did pretty well but had to be careful. Since I have a bunch of sheetmetal to weld in some future projects (at least two floorboards,rockers, and maybe a chopped top), I bought me a second mig and set it up for just .023....which means less heat input with each stitch. Stitching is like a line of overlapping spotwelds. It seems to work well for sheetmetal and somewhat limiting heat input and subsequent warping. Controlling the heat input is a major concern when welding thin sheetmetal. Doesn't hurt to pause between each stitch to allow the heat to dissipate.Also, grind welds slowly and don't heat the panel with the grinding wheel either. Go slow and allow cooling. Try to never get the panel or the weld red.....just keep it cool and everything will be cool..........
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