How To Make A Patch Panel
Article by Mark Trotta
If an automotive body panel has rust spots but no collision damage, replacing the whole panel is not always necessary.
An alternative is to cut out only the rusted area, then make a template, trace it onto sheet metal, cut it out, and weld it in. This is a time consuming process, but a little planning and having the right tools will save you a lot of time.
Full Panel vs Patch Panel
If you bring your car to a body shop, they'll want to replace the entire panel, regardless of whether or not it can be repaired. This is simple economics--it's quicker and more cost-effective for them to replace the entire thing. The labor may be less, but once you include the cost of the replacement panel the total bill will be more. This why farming out bodywork is usually out of a person's budget.
Provided you are a decent welder, you can save money by cutting out the bad metal, making a patch panel, and welding it in.
If you're just getting started with automotive body repair, a MIG welding machine is a good choice. It's the easiest welding process to learn and also cheapest to buy. Most people can produce quality, good looking welds with minimal practice.
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Getting Started
You need to bring the panel down to bare metal. A D/A sander works best for this.
An angle grinder with a wire brush attachment can also be used.
Before cutting out the rusty parts, take a wire brush and/or scraper and remove any loose rust. Then, take masking tape and mark out one inch beyond the rusted-through part.
You want to remove as little good metal as possible.
Cutting and Removing Rusty Panels
Cutting out damaged sheet metal can be done several ways.
An air compressor running a cut-off wheel is one popular method.
Once the rusted part is cut away, an air sander can be used to bring the area down to bare metal.
Sand beyond the rusted area an inch or so. A 36-grit sanding disc works the fastest. Clean the exposed area with a 60 or 80-grit disc. You should now have a shiny, solid rust-free surface.
Patch Panel Template
After carefully cutting out all the rust, 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 and cut a template of the patch panel needed.
Start larger than you actually need, then slowly trim it down.
Next, scribe or mark the cardboard template onto a piece of sheet metal. You can use a black marker to mark the cut, and remember to add about an 1/8th" or so to the line for trimming.
TIP: Trim slowly, little by little--cut and check fit. You want to end up with a gap of no more than 1/8".
I prefer to use hand-held snips, but sheet metal can be cut in a number of ways.
Read: 9 Ways To Cut Sheet Metal
It's easy to remove metal - much more difficult to add it!
Butt Weld vs Lap Weld
The best way to weld sheet metal panels together is by butt-welding, which is joining them side-by-side.
A lap weld is much easier than a butt weld, but doesn't seal correctly, and will promote moisture from the back side. An overlapping joint will provide a place for moisture to collect and rust to take over again.
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 concern.
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Best Metal For Patch Panel
You can buy a new piece of sheet metal stock, or use an old piece of panel from a donor car, as long as it's grinded clean and of the same thickness.
How To Measure Sheet Metal Thickness
You can measure sheet metal thickness with a digital caliper or a sheet metal thickness gauge.
NOTE: Most classic American car body panels are 20 or 22 gauge. On pre-1970's cars, the width of the original sheet metal is often thicker (18 or 20 gauge).
Curved Patch Panels
Patching into a curved area of the car body will take more time and skill than 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.
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