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Old 01-20-2020, 01:22 PM   #1
rgunlock
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Patching a large panel - need advice

Got this truck that had the auxiliary tanks at some point. I am trying to patch the holes on each side since I don't have the tanks anyway.

I just started on this patch and I'm already getting a vertical low spot from about the middle of the patch up to the body line with the moulding. I checked and the panel was flat before I started. I am only welding 2-3 spots per side, then grinding off the tops of the weld. Later I come back and do a new spot adjacent to the previous ones. Along the top, they've been clean welds without burnouts, so I didn't think that the heat should have been excessive.

The only thing I can think of is that I have not been able to planish the welds because its pretty tough to reach behind. So, any clues as to what I'm doing wrong, and more importantly how I fix this before it gets worse?

Thanks, Rick
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Old 01-20-2020, 02:25 PM   #2
mongocanfly
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Re: Patching a large panel - need advice

id say hammer and dolley is needed
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Old 01-20-2020, 02:35 PM   #3
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Re: Patching a large panel - need advice

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id say hammer and dolley is needed
Turns out on this side I can get a dolly up behind it and I have planished (beat on) all the welds I've done. That still hasn't straightened the panel back out, but maybe its a little better. Do I just hammer more on the welds than I have?
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Old 01-20-2020, 08:47 PM   #4
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Re: Patching a large panel - need advice

I was hoping Robert or John would chime in...I think the metal shrunk when you welded it and needs stretched back out...with hammer and dolley...I had the same thing happen when I welded in the IH cab corner..it pulled in on me..but there was no way to get to the backside..it was only 1/8" or so....so I just fixed it with filler...not ideal but it was all I could do
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Old 01-21-2020, 11:09 AM   #5
rgunlock
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Re: Patching a large panel - need advice

Thanks Greg! The patch on this side (shown) did turn out better than the first one I did on the other side. There is still distortion but it doesn't spread nearly as far. The other side has a big vertical depression both above and below the patch.

I'm wondering if I cut out the patch and perhaps an additional 1/2 inch around the edges of it, perhaps it will relieve the stress on the rest of the metal and I can just try again.
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Old 01-21-2020, 02:20 PM   #6
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Re: Patching a large panel - need advice

Anytime you weld you will see shrinkage (compared to original) as the weld cools. Doesn't matter if it's the 19 gauge sheet steel from the factory or 1" thick plate, it all shrinks as the metal cools.

Most body panels have a certain amount of crown in order to hold the panels shape without flopping in the breeze. your quarter definitely has a crown in the vertical (up-down) and likely has a slight amount horizontally (front-back) See picture below....


Where the red shows the original crown of the panel, any shrinking due to unchecked welding (no planishing to stretch it back out) will result in the weld shrinking in length, or an arc moving toward becoming straight/straighter (blue line). This is exactly why you show a low after welding. Given the difficulty in planishing due to location, I can see where your efforts may not be as effective as an area easier to reach. Anytime you are planishing you should hear an on-dolly "ping" when striking against the dolly to ensure effective stretching. Sometimes it helps to hammer in the direction the panel needs to go, with hammer on the inside, dolly on the outside. Keep in mind the hammer face should be measured just as you would a dolly, close to the crown's shape as you can get without the edges touching. This helps eliminate any errant marks from the edge of the hammer hitting the panel.

Here also I'll mention that with welding MIG "dots" you should have a full penetration weld such that you have weld proud on both the front and back. This makes it easier to get a stretch as the dot gives you a "target" on both sides without hitting the adjacent sheet metal. So weld the full penetration dot, planish while it's easier to strike, then grind just above flush ON BOTH SIDES so it's out of the way of your next weld dot for planishing. Weld, planish, grind, repeat. Just like the shampoo bottle.

Last comment, any welds in a corner will focus heat on the inside corners that you will see more shrinking on the inside, for a pucker in the inside corners. This makes it a bit more difficult to remove the distortion. For what you are working with a round replacement patch or one with radius corners helps to balance shrinking effect inside to outside of the weld better than using sharp corners. (If your tin snip skills are up to par and you can cut accurately for both hole and filler panel) This video is a good tin snip refresher/training aid for anyone and everyone. Thought I knew all there was to know about using tin snips and this showed me what I was doing wrong..... for many years.

https://www.youtube.com/watch?v=qAOQfUaRZyw&t=23s



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Old 01-21-2020, 03:17 PM   #7
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Re: Patching a large panel - need advice

Thanks for the tips Robert! I was able to planish the welds on the left side of the bed because I could squeeze the dolly up there with my left hand and hammer with the right. Beat heck out of my knuckles though against the inner bed panel! No tool that I have would fit up there to grind the backside of the welds though. On the right side of the bed I'd have had to hammer left handed, which would have turned out badly I'm sure. That's where I ended up with the worse distortions.

Question is, is the surrounding metal likely to be shrunk, say 1/2 or 1" out from my weld, or could I cut out my patch and expect the panel to straighten back out? Of course I'd still have to come up with a better plan for the retry.

Edit: just watched the cool video. I never thought about right vs left snips, but would have assumed they were for right versus left handed people. Lot of good info there - thanks!
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Old 01-22-2020, 04:20 PM   #8
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Re: Patching a large panel - need advice

Quote:
Originally Posted by rgunlock View Post
Question is, is the surrounding metal likely to be shrunk, say 1/2 or 1" out from my weld, or could I cut out my patch and expect the panel to straighten back out? Of course I'd still have to come up with a better plan for the retry.

My fear is that without adequate room to planish for removing the shrink effects, doesn't matter how many times it is done, it will still shrink. Either you can get to it or you are relegated to filler for masking the distortion. For really ambitious, the bed side could be removed, weld in and planish/metal finish the patch, and then re-install bed side.
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Old 01-22-2020, 04:40 PM   #9
rgunlock
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Re: Patching a large panel - need advice

Yep, I was afraid that would be the answer. There is access to the backside, just not using my left hand so I can hammer with the right. Been trying to figure out a clever way to hold the dolly back there with enough leverage to be able to hammer against it. Would be tedious, but maybe with a wedge... Maybe the best way to find out whether the surrounding metal will pop back into shape is just to cut out the patch and see.

Thanks for taking the time to respond!
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Old 01-22-2020, 05:22 PM   #10
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Re: Patching a large panel - need advice

Before cutting something back out, perhaps enlist the help of someone else that can hold the dolly so you can hammer right handed.
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Old 01-22-2020, 05:42 PM   #11
rgunlock
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Re: Patching a large panel - need advice

I'll see if I can find a volunteer BTW, I have been following your 55 Wagon thread and many of your other posts. Whatever I'm actually doing right is largely because of the detailed how-to information you include in your posts. Much appreciated!
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78 GMC K15 SWB, 350/NV4500/NP205/4.10s Project Thread
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Old 01-22-2020, 09:01 PM   #12
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Re: Patching a large panel - need advice

Glad to help out!
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