I seen an article in Car Craft regarding using a torch to shrink metal by heating it up, then immediately applying a wet rag to it. I just finished tack welding up a lower patch on my fender that I made from a chunk of quarter panel from my parts car :rolleyes: . I originally had a 1/16" gap all the way around, and jumped around so not to apply too much heat in one area. Anyway, the fit seemed to change as I went, I'm assuming because of the heat, but it seemed to expand, and got pretty tight in there. After I was done, I used a straight edge and noticed there is now a high spot across the middle of the patch. I wouldn't think I could gently beat it down because the metal wouldn't have anywhere to go. It's about an 1/8 high in the center, so can I use the shrinking method, or will I run the risk of warping it really bad. Thanks for the help!

Any ideas on this? Thanks.

Here is the basics of shrinking for you. Did you weld this with a gas torch or a MIG? Did you do any cooling as you welded? I doesn't sound like you did and hopefully you didn't. If you do want to shrink and don't have access to the back you will need to heat and cool. I'll tell you, using air is much better than water if you are only doing small shrinks. Water rusts the metal right NOW and it is a mess. Heat a very small area like the size of a dime not red hot, but close, then cool it with air using a blower right up on the metal.

If you do very small shrinks along the seam where you welded you can keep it under control. By the way, the gap was one of your problems, you want a tight fit if you are butt welding.
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"Basics of Basics" Shrinking. By Brian Martin

Where is the dent and how big is it? These may be deciding factors in how you repair.

If your panel needs is shrinking. You can do this in many different ways, again
depending on where and what size the dent is.

You could use:

A torch. This can provide the most heat for shrinking, consequently the most DAMAGE.

A shrinker attachment on a spot or MIG welder. This is a great way to shrink small dents
or thin metals.

A grinder. Yes, with an 80 or 100 grit disc and a lot of speed you can heat the high spot
and cool it to shrink, without taking "much" metal off.

A "DA". With it in the "grinder" mode and some 120 grit, use it the same as the grinder. I
do this all the time, it is very useful.

A Heat gun. There are electric hand held heat guns that provide you with 1000 to 1500
degrees of heat.

Simply use an “off dolly” technique with a hammer and dolly. Push up on the panel with the dolly, then tap around the area OFF the dolly. Many times there will be high spots anyway around a low spot so this works perfect. Remember, do not hit the hammer where the dolly is. That is called “on dolly” and STRETCHES the metal.

There are different methods for shrinking, you heat and cool or you heat and work.

Heat and cool is usually for areas that you can't get behind, very small low spots, or very
thin metal. This is the easiest way in that you just apply heat and then cool with air or
water and the area will be shrunk. Cooling with air really works well. I have done some
little tests and blowing the heated area with a blower on your air hose shrinks almost as
much as quenching it with water, without the rusting concerns. When the metal cools the
molecules get closer together in the heated area, thus pulling in on the surrounding area
and shrinking the surface area of the panel.

Heat and work is a bit trickier. You heat (usually with the torch) and put a dolly behind
the heated metal and gently strike the heated area with a hammer “On Dolly” (the largest,
flattest hammer you have). When the metal is hot the molecules are free to move about. So
after heating and before the metal cools, tapping on the heated area (that has either raised
or dropped) allows you to “push” them to where you want them.

You have to do this carefully, because if you hit “On Dolly” too hard, you will push those
molecules apart, and make matters worse! The idea is to gently push the molecules to the
center of the heated area and this will “pull” in on the surrounding metal. Picture a 12 x
12 inch 1/4” think tile of playdough. If you maintain the 12 x 12 but thin the Playdough
down to 1/8” you would have a big hump in the middle right? Well this is what the metal
is doing, you have to move the molecules like the particles of playdough back to the
proper place. When you heat that sheet metal, believe me you don’t have to hit it much
harder than if you were hitting Playdough, so be CAREFUL!

Precautions:

Bare metal is the best to shrink, especially the back side. BE CAREFUL THAT THERE
IS NO UNDERSEAL TO BURN, and besides if you are going to “work” it cool, the
dolly gets all covered with underseal.

Get a partner to hold the torch while you “work” the metal. And this partner can also keep
a fire watch.

Make sure you know what is behind the metal you are about to heat, wires, lines (like
FUEL!) should be removed. And that sound deadening material can be VERY flammable
(I know all to well about that one!) .

The metal will only do what you tell it to do. It has a memory and you have to
“help it to remember”.

Thanks MARTINSR! I did find this post on a search...after I posted my question of course! OK, so I should search and then post if need be, got it. graemlins/clonk.gif

I saw an episode of "a car is reborn" (where that english guy rebuilds the jag) where they shrunk a high spot.

They used a similar technique to the "heat and work" method described above, but they just heated the high spot cherry red and hit it with a flat piece of metal (no dolly behind).

I don't know if one method works better or is easier for the inexperienced.

Originally posted by cperrell:
I saw an episode of "a car is reborn" (where that english guy rebuilds the jag) where they shrunk a high spot.

They used a similar technique to the "heat and work" method described above, but they just heated the high spot cherry red and hit it with a flat piece of metal (no dolly behind).

I don't know if one method works better or is easier for the inexperienced. That is simply the "off dolly" method I explain in the text.