There are two ways to repair rust on body panels - cut out the rusted section 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 cutting out the rusted metal and welding in a patch panel. Provided you are a decent welder, you can save money and repair rust on body panels yourself.
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. Remember how a good pool player sets himself up for the next shot? That's what you're doing here.
Using The Right Tools
Having the right tools makes all the difference, provided 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, also known as a cut-off wheel. A die grinder 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 your 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.