I have had a far amount of success welding patch panels on this rust bucket of a panel truck. I started on the floor pan, then the inner/outer cowl, also welded in a piece of rotted out tool cubby under the driver seat riser. So I thought I was ready to weld on the lower patch between the back of door and rear fender. Turned out to be the worst panel yet. I have a couple spots that are convex and another are that is concave, also the area that is concave has kind of warped the area above it giving it a kind of oil can effect. I have tried to tap the high area down but with no way to get a dolly behind it my efforts were futile. This is the method I have been using to weld so far with success until now (using a hobart handler mvp 220v #2 setting): I tack welded every inch or so then smoothed out the tack welds, then skipped around filling in between the tack welds, checking to make sure everything cooled before next weld. Any advice on what I may be doing wrong so I can get it right on the other side. Don't know what I can do about drivers side, thinking about cutting it out and starting over! I have welded this in with a slight gap about 1/32 between panels I have read conflicting views on this on the web some say to butt the two pieces together others say to leave a small gap as the metal will expand. Which way is correct?
How far up on the quarter did you weld? The farther you go up, the lower the crown of the panel. Welding in low crown panels, with the shrinking the metal experiences from the heat of welding, is quick to loose it's shape and turn into a floppy mess (oil can) as you describe. Leaving a gap between the panels as some suggest, only allows for more panel movement due to the shrinking effect of the welding heat and nothing to prevent it. Ever weld on either side of panel clamps only to find them difficult to remove as the panel has shrunk around them? Case in point of the gap allowing the panel movement. This in turn will compound the flattening of the panel crown/oil can results as you are seeing. Where the panel will expand slightly as it heats, the molecular structure of the metal will realign tighter as it cools, thus shrinking. As a long linear weld shrinks, it has a tendancy to gather/pull the metal immediately outside the weld. A bit farther out, the remaining panel will have a tendancy to resist this movement. In extreme cases, the area in between the two will create a wave in the panel, and many people will misread this movement as the panel stretching. As an example, take a piece of paper and lay flat on a table in front of you. Hold one corner down with one hand, and using the other hand, push the adjacent corner towards the one you are holding. Moving the corners together will simulate the linear shrinking of the weld, and the resulting wave in the paper that results is what happens to the adjacent panel that is resisting the movement.
My preference is to fit the panels with absolute tight joints (or to the best of your ability) to minimze this panel movement. When using a MIG, use weld dots skipping around every so many inches. Before getting too crazy, insure that you have a full penetration weld with a slight bit of weld proud on the rear side. If you still see two separate panels on the rear after tacking, you have insufficient weld penetration that, given grinding the welds from the outside to produce a flat panel, will likely fail down the road. Turn up the heat if you need more weld penetration. If this tends to produce blowouts, turn up the wire feed a bit, as more heat and not enough filler results in burning the surrounding panel.
Next, after each weld dot, grind the front and rear of each weld proud to just ABOVE that of panel surface. Then using your hammer and dolly, planish each weld to stretch it to overcome the shrinking it experienced during the cooling. Next, some will suggest to add dots in between until you have a solid weld (keep grinding and planishing after each weld..) but I would recommend to instead overlap each previous weld dot by approx 1/3 to 1/2 dia for the next weld placement, as this will eliminate the problem of missing or leaving a gap between any weld dots.
By planishing each weld dot as you go, it will minimize the shrinking effects from welding, helping to eliminate any oil can effects. Then, any metal bumping required afterwards in order to straighten out the panel should be minimal.
Getting good penetration but I have limited access to the backside of panel, only 4 approx 1 x 8 access panels all the way on the top of the inside of panel bed not allowing for any normal hammer and dolly work. Guess will try to butt the pieces together next time, will make welding alot easier. The gap worked well for all my other welds but they were nowhere as big as this one. I guess there is alot of misinformation out there to leave a gap to eliminate problem of expansion. So what do you think cut it out and see if Ray(very busy) can make me another. Or can it be saved?
While MIG welding is likely the most prevalent used by the home hobbyist (as well as many businesses) it isn't the most user friendly in preventing damage to sheetmetal panels as you are experiencing. Some thoughts: (actually a lot of thoughts...)
Any welding operation is going to cause a certain amount of shrinking. Welding in a square-ish repair panel (for example) toward the bottom of a fender will induce a certain amount of shrinking and distortion, in four directions. By extending a patch to the wheel opening, and all the way to the bottom of the fender, you eliminate two weld seams, and half as much shrinking. If you can replace the complete lower fender from wheel opening to door opening, to the bottom, now you are down to one weld seam, and all the forces are pulling in a similar direction (as opposed to what you see with two intersecting welds) and the distortion should be less.
If you can trim your joints to zero gaps such that a no-filler weld (fusion weld) can be performed (Using TIG or O/A), then you should have about as small a HAZ as possible, and as consistent in width as possible, for less distortion. To explain this further, various starting and stopping in your weld will cause inconsistent width of the HAZ, along with the shrinking effects that come with it. This may lead to a "wavy" distortion. Consistent heat, consistent speed, without stopping, in performing a fusion weld will help to keep the HAZ, and all the other conditional reactions consistent, for less distortion. Having said all that, this example was for use with a TIG or O/A. Most enthusiasts will be using a MIG welder (as that's what they have) which is not very compatible with a full pass weld when dealing with sheetmetal. But you can still apply the same principles of consistency in using a MIG and "dot" welding. Single dots, skip around, use same overlap, same size dots (elapsed time of trigger pull) etc. This won't make the weld as nice as the fusion weld above, but it will help with consistency. And the biggest thing is to practice on some scrap pieces the same guage as you'll be working with to insure your welder is set up correctly for a full penetration weld. It's hard to be consistent if you're putting in the patches and still fiddling with welder settings, (practicing on the good stuff).
Where the TIG or O/A fusion welding does require some additional skill/dexterity over the "point and shoot" of the MIG, it by far is the best method to minimize damage to panels. Done correctly, typically the weld on the back side is nearly flat with little if any weld protruding form the back side. If there is weld proud to the back side, you will usually see a slight depression on the outside, that can easily be raised with minimal hammer and dolly work, flattening out both sides simultaneously. This in itself eliminates much of the grinding effort required to dress out welds like when using the MIG. Where the fusion weld may not be a viable option for you due to eqpt availability/skills, this hopefully will impress the importance of consistency in whichever method you use.
Short of having the ability to use the fusion welding method to minimze damage, I would recommend either using repair panels that will come up far enough to allow access from the back (ie: above the inner panel) for hammer and dolly work, or opening up the inner panel to allow this access. This would need to be done with some good planning to make closing it up later as painless a process as possible.
Here is a thread that shows using the fusion weld process. I hope to get to this point in the future, but still need to get past the teaching old dogs stage....
A video to show the TIG fusion welding process, panel was originally tacked together using the Tig (tacks are barely visible) before performing the fusion weld. Take note towards the end how consistent the HAZ is on either side of the weld.
