1/3/2006 Rev. 1


Metalwork- Better ways to patch those nasty rusted panels.

By now you have figured out I have done quite a bit of rust repair. This all started back in college when I did the rust work on a 66 Mustang Fastback. It was rusty, but not as rusty as then can get. I did the floors, lower quarters, radio section of the the dash, and so on. I started with the floors figuring how bad could I mess them up? I seemed to do ok at MIG welding scrap, it did not fall apart. Keep in mind I am a self taught welder. So I did my best to measure and cut and and rig and then I go to put in the floor pans. Of course the repro pans are designed for a convertible (and just placing under the original metal) so the edge was turned down and not up- UUGGGG! I had to rework the edge.

So about 10 feet of welding and grinding later. I go to MIG weld upside down. The molten metal just comes back into my face. I played with the settings and magically beautiful welds upside down with great penetration. Now I go and look closer at all my other welds, NO penetration. So another 10 feet of welding at the new settings and I have full penetration now.

So the first thing you must understand is how to weld with full penetration. This is a very common problem that I have frequently seen others guilty of commiting.

When I had started with my cabriolet I had this great idea of doing all the body work to the point of not needing bondo to level it. I had found some info on the web and had noticed some of the pros were all using oxy-act to put the panels together. I also knew that you had to do some hammer on dolly work. I figure this should be easy. So I make a patch up from scratch for my cowl and weld it in with the oxy-act. Well that just did not work very well. After some playing I sort of had the weld flat and I had a nice oil can. The wives tale we all hear is- Shrink the oil can. I chased the oil can all around the panel. Well nothing some bondo did not fix in the end.

Things started changing after my cowl patch problems. I had been doing some read and bought some videos. I also re-found and autobody vo-tech class text book from the early 1970's. My body work started to improve as I started to understand more about moving and shrinking metal. I watched the video, re-read the autobody book and go out and play for a while then come back the re-read some more. Stuff started clicking and I started to get what I was doing. Things started going better.

Let me try to put some concepts in you head.

Take a 8.5 X 11 sheet of notebook paper (or what ever)

Tear about 1/2" into the long side in the middle.

Now, slightly overlap the tear and note what just happened.

The paper will bulge up or down on you.

Now I ask you, How do you fix the problem??

Well you could try to shrink the high spot down. But it that the real problem?

The real problem is the edge is now too short and needs to be stretched.

Now lets think a little about curved panels.

Take a drink can and push in the side. This, I believe is called a rolled buckle.

Note the top of the can now is shorter on that side.

So curved panels with damage can cause shrinkage.


Keep the two above examples in mind as you read on.


A basic rule of Metal: ANY time you heat metal past a certain tempurature (where it steel starts turning blue) you create a shrink point.

Shrinkage can cause all kinds of grief and may not be obvious in the area that you are working. Keep in mind a small shrinkage may be pulling on a rolled section of a panel which in turn causes a distortion quite a distance from the actual weld!

So how do you fix weld shrinkage? Easy, you need a hammer and a dolly and you need to do hammer on dolly hits in the area affected by the heat. For some welds you need to first grind the weld smooth. This is sometimes called hammer welding which is not a correct term as you hammer weld in blacksmithing, you have already done the weld. You need to planish the area smooth.

Those are some basic facts you need to keep in your head.

When you go to install a panel keep in mind you can not planish the weld if you do an overlap weld. I see instructional videos and website where they show doing the overlap and call it easier to do. Well butt welding is not really that hard and with a MIG welder it is actually fairly forgiving when you have a gap. So I do not really see it as a time saver and it will keep you from being able to planish the weld area. Plus if you do not properly treat the overlap after welding you will have this nice area to start rusting again. When you buy patch panels keep in mind they are far from perfect. Most are flat and have a poorly rolled in bead that does not contour like the original. The patch panels should always be somewhat pre-formed to the curvatures of the panel. The door is a good example. They curve top to bottom and front to rear. You need to beat the patch panel to be close (not perfect mind you) to both curves. This makes a big difference in how well the patch fits when you are done welding. It also means much less work trying to get the contours right with bondo when you are done.

Some Welding Tips.

Copper. One word say a lot. Various chunks of copper are great for backing MIG welds. They suck away some heat and they are great when filling a gap. Brass does not work, the tin leaches out and causes problems. I have even formed some copper to get a basic shape.

If you need to fill small holes use a regular TIG or oxy/act rods along with the MIG. Put the rod in the hole and light the arc off on it. Keep feeding the rod in at the same time. This will take away the high heat that causes burn back and lets you fill quickly. Try it once and you be loving it.

Rusty thin metal is hard to weld. Cut back to good metal if you can. If you are cutting a panel one or 5 inches is still the same amount of work to repair. That is if you are lucky to have good metal some place, my cabriolet has some places I really did not want to weld to. Rust in pits will cause a neat little volcano thing when you go to weld them. This is very porus metal, try to sandblast before you weld.

Practice practice practice. The question comes up what kind of welder to get. I say get the most power and wire feed settings you can afford. For MIG you really want a name brand 220V welder with gas. But I know budgets do not allow such things. I have seen some nice welds done with cheap 110V flux core welders. They will be slow and you must that some time to learn how to do it.

Stitch weld. You want to do a spot weld and move to a different section and do another spot weld. You than slowly connect the dots being carefull not to put too much heat into one area.

If you can afford it, buy a TIG welder. There is nothing more satisfying than doing a patch for the first time and having 2/3 of the weld not needing any real grinding. That was the first patch I put in with my old huge Miller 330a/bp. They can be found for around $600 ready to go, but they are big, heavy and such current. They are great for welding big things and can do sheet metal too.

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