This is a tricky situation depending on the extent of the rust; is it just on the surface of the body, or has it turned to cancer and eaten through the car; the surface rust can just be ground off and the area repainted and that is quite simple.
What you need to do to find out if the rust is all the way through the body of the car is just use a screw driver and lightly try to push it through the body in a few places around the rusted area, if you can push it through the body of the car it's time to fix it.
Now you need to determine how big the affected area is, and they can get really big really fast, so don't expect something the size of a quarter, to determine the size of the rust spot on the car we'll use a body hammer and lightly tap around the affected area.
You'll be listening for a difference in tone, and feeling for a difference in rigidity of the metal, a rusted area will have a lot of flexibility, and a god area should be pretty solid remember the key here is lightly tap around the affected area.
What we're looking to do is create an area of good metal that is at least two inches wide around the affected area of the metal, once you know where the affected area is you'll need to take a Sharpie and trace around that area.
Once you have traced around the affected area, mark an area about two inches out from the affected area all the way around it, this is where your going to make your cut to patch the car, don't through t the piece that you cut out away, you'll use it as a pattern.
The next thing is to make sure that you have the right gauge of sheet metal, this is very important, you do not want a piece that's to thick or to thin, this can and will make uneven areas in the body of your car.
When you go to buy the metal to replace the bad area ask the place that your buying it from to measure the gauge of the sheet metal for you, and make 100% sure that you get the same gauge of metal, after you buy the sheet metal, you'll be making the patch panel from it.
What we're going to do here is lay the piece of sheet metal that you bought on a bench or on the floor, and then take the rusted piece that you cut out of your car and lay it on top of the new piece, get out your Sharpie and trace around the pattern of the metal.
Now you'll need to decide what your going to use to cut the patch panel out of the metal, it doesn't matter if you use a die grinder, hand shears or some other tool, use what makes you the most comfortable, but take your time doing it.
Now you'll cut out the patch panel using whatever tool that you chose, you'll need to stay as much on the outside of the pattern line that you drew as you possibly can, once you've cut the patch panel out of the new piece we'll be done with the cutting part.
List of Tools Needed:
- Auto Body Hammers
- Die grinder
- Hand shears
- Air compressor
- Mig welder
- five inch air grinder
- Panel clamps
Above is a list of must have tools, if you don't have some of them it's best to let somebody who does do the job, if your in a position to go out and buy the tools that's great, you will need all of them though, this job cannot be done without them.
Now we'll be fitting up the metal patch that you made to the hole in your car, if it's to big in a few areas we'll use a five inch air grinder to make it fit, this does happen sometimes if it did this time all we need to do is grind the areas that are making it not fit.
After you have done the fit up, you'll notice that the back edge of the patch has rough areas around the outer edge, you'll need to grind these off with the five inch grinder, now we can begin putting the patch panel in the car.
You'll need to make sure that you have the vicegrips ready to go, and a mig welder ready to go, you'll be using the vicegrips to hold the patch panel in place, in areas where vicegrips won't work you can use a panel holding system, or panel clamps.
Now it's time to put the patch panel in place using the vicegrips, or the panel clamps, once you have it new panel in place you need to make sure that your mig welder is ready to go, use a piece of scrap metal to get it set the way that you like it.
My self I usually match the wire speed with the amp setting but sometimes yo need to slow it down, or speed it up a bit to make it work right, the settings are something that you'll have to play with to make it comfortable for you, always use a welding visor.
A word of warning, you should never in any case try to run a solid bead all at one time around the entire panel, what you need to do is run about one inch beads at a time, one at the top to hold the panel, then one at the bottom of the panel.
I usually cool them with air as I go to limit the possibility of warping the panel, you should keep to the idea of welding one inch beads at a time as far from each other as possible, this will limit the possibility of warping the new panel, or the sheet metal in the car.
Id your keep this up eventually you'll have a weld bead all the way around the patch panel, then all you have to do is use the five inch grinder to smooth the welds out on the car, if you've done it right they should almost disappear from the car.