Repair Rust On Body Panel
If a hood or trunk lid on an old car needs repair, they can generally be removed and replaced easily. This goes for doors, too, but to a lesser extent. Panels that are an integral part of the car will require welding.
Old cars usually see rust and rot on fenders, doors, quarter panels, trunk floors, and interior floor pans. Back when I lived in the Northeast, I repaired a few rotted dashboards and kick panels too. Damage from rust and rot varies, so every repair will be a little different from car to car.
If you're just getting started with welding, a wire-feed (MIG) unit is a good choice. They're easy to learn on, cheapest to buy, and most people can produce quality, good looking welds with minimal practice.
read Best Welder For Automotive Projects
There are two methods to repair rust on 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 may not be necessary. An alternative is to cut out the rusted area and weld in a patch panel. Provided you are a decent welder, you can save money and repair rust on body panels yourself.
read MIG Weld Sheet Metal
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.
read Best Air Tools For Automotive Restoration
Before cutting out the rusted metal, clean off the area. Take a scraper or putty knife and scrape off any loose rust on the body panel to be worked on. Then take masking tape and mark out about one inch around the rusted-through part. You want to remove as little good metal as possible. The big key here is to think it through before cutting.
Use an air sander to quickly 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. Once the damaged metal is removed it's easier to see the best way to make the patch.
Patch Panel Template
After carefully cutting out the rusted metal from the panel, the next step is to make a patch template. Take measurements and transfer them to a piece of 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.
Making A Patch Panel
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. On older (pre-seventies) American cars and trucks, the width of the original sheet metal is most likely 18 or 20 gauge. Carefully cut out the metal template, checking and re-checking it against the car for an exact fit. You want to end up with a gap of no more than 1/8".
Cutting Sheet Metal
Hand-held nibblers and tin-snips work fine for smaller patches, but electric shears will save time on larger patches. You can use a black marker to mark the cut, remembering to add about an 1/8th" or so to the line for trimming. Once you have the rough patch cut out, carefully trim it down with hand-held snips.
Curved Patch Panels
Patching into a curved area of the car body will take more time and skill and a simple flat patch. Before cutting out the old metal, study the damaged area and think through the repair process. You may decide to remove the entire section to make the patch and welding more accessible. Cutting the panel bigger than the damaged area may give you a cleaner and easier butt weld.
With the panel shaped, test fit it to the car body several times, then clamp it down. Welding new to old rusty metal is always tricky, especially after grinding the rust off old sections, which thins the metal even more. Burning through the old metal is always a worry. The following article covers tips and techniques for welding sheet metal.