Repair Rust On Body Panel
There are two methods to repair rust on steel body panels. You can cut out the damaged section and weld in a patch panel, or replace the entire panel. Most body shops will replace the entire panel, simply because it's quicker and more cost-effective. The labor is usually less than it would be to repair the rust, but you pay more for the replacement panel. This often puts the job out of many people's budgets.
If there is no collision damage, replacing the whole panel is not necessary. A good alternative is to cut out the rusted metal and weld in a patch panel. Provided you are a decent welder, you can save money and repair rust on body panels yourself.
Cutting and Removing Rust
Cutting out the metal to be repaired can be done in several ways, but before you start grabbing tools, stop and think about the best plan of action. Having the right tools makes all the difference, providing that you use them correctly. An air compressor and air tools will save you time and allow you to work more effectively. The most basic air tool for bodywork is a die grinder, which serves two purposes. It can be used as a cut-off wheel, and by changing the arbor, it becomes an air grinder.
Before cutting out the rusted metal, take a scraper or putty knife and scrape off any loose rust on the body panel to be worked on. Take masking tape and mark out about one inch around the rusted-through part. You want to remove as little good metal as possible.
Use a die grinder to bring the rusted area down to bare metal. A 36-grit sanding disc works best. Sand beyond the rusted area an inch or so. Once down to bare metal, all the damage is revealed. Clean the exposed metal area with a 60 or 80-grit disc. You should now have a shiny, solid rust-free surface.
Again, take masking tape and mark off the intended cut area. You can use tin-snips, or a coarse file to remove the rusted metal, but an air-powered cut-off wheel is fastest. Slice along the taped line with the cut-off wheel, letting the air tool do the work. Don't force it to cut faster, as this will overheat the tool and the metal. This will make a lot more work for you.
Making A Patch Panel
After carefully cutting out the rusted metal from the panel, the next step is to make a patch template. Using a piece of light cardboard (the kind you can cut with a scissor) draw out and cut a template of the patch panel needed. Start larger than you need, then trim gradually until a perfect fit is achieved. After that, scribe or mark the cardboard template onto a piece of sheet metal.
Patch Panel Template
If you can't purchase a new piece of sheet metal stock, an old piece of panel from a donor car can be used, as long as it's grinded clean and the same thickness. Carefully cut out the metal template, checking and re-checking it against the car for an exact fit, with a gap of no more than 1/8". Hand-held nibblers and tin-snips work fine for smaller patches, but electric shears will save time on larger patches.
read MIG Weld Patch Panel