All the paint in the world won’t make a vehicle look any better if it’s applied over metal that is flaking away due to corrosion. Project F-250 has rust holes around three of the wheel wells, so tackling that problem is next on the list.
There are several companies that sell replacement fenders for trucks at a pretty reasonable price. Because Project F-250 had rusted sections on both front fenders, the easiest option would have been to call one of those suppliers and get new ones. But the object here is to keep costs down, even if it means spending extra time in the shop. So the worst rust spots will get cut out and we’ll use metal from a scrap fender left over from another project to make weld-in patches.
Cutting sheet metal is pretty easy, and there are a variety of tools you can use to do it. On this project we used an air powered, rotary cut-off tool. After sanding the area down to bare metal so it was possible to get an accurate look at how extensive the rust was, the area to be cut out was marked off using masking tape as a cutting guide. You could also use a “Sharpie” felt marker; they work well, too.
Make sure to cut all the way back to good clean steel, otherwise you’ll find it impossible to weld the patch in alongside thin, rusty metal. On the right front fender we could have cut out a larger section of the panel, because there were other rusty segments nearby; but they weren’t too bad; so we chose to treat them with rust inhibitor, instead. That also saved us some time.
Using the removed, rusty sections as templates, their shape was traced onto other pieces of good metal and replacement patches were cut out using the same air tool. We’re going to butt weld the patches in, so the new pieces will have to fit perfectly, without any gaps. If you use this method, it’s better to initially leave the patch a little large and gradually trim it to fit. Where you have gaps of 1/16 of an inch or more, you’ll find it pretty hard to butt weld sheet metal.
An easier way to weld in a replacement piece is to use an overlapping joint. Just leave about a quarter inch of extra material on all sides of the patch and fit it in behind the hole in the truck fender. Getting it in between a double-sided panel will involve a little fiddling, but it’s possible. Drilling holes through the panel and the patch will allow you to old it in place with sheet metal screws while you weld it. You can then remove the screws and weld the holes closed.
To weld sheet metal, the simplest method is to make a series of tacks all around the joint. Do this until the patch is completely attached and all the tacks have formed together into one continuous weld. Using short welding bursts rather than trying to run a continuous bead prevents burning through the thin metal, which is easily done.
Go slow and work all around the joint rather than concentrate the welding in one area. That will minimize heat distortion, too.
It’s best to practice your welding technique before tackling a project like this. You’ll need a MIG welder. An arc type just can’t do the job. A welding helmet with an auto-darkening lens will make things easier.
For Project F-250, we used a small, 90 amp MIG welder loaded with 0.023 welding wire and an argon-carbon dioxide shielding gas mix, which is common for most MIG applications. The narrow gauge wire keeps the welding temperature low. Larger diameter wire burns hotter, which makes it easier to melt through the thin metal or create heat distortion in the panel.
Once the patch was in, the weld was ground down until it was even with the surrounding material. A coat of fibreglass filler was applied over the entire patch area and sanded smooth to hide any minor imperfections. A coat of primer was sprayed over top of that. Now, the fender is fixed and ready for paint.
For this phase of the project we had to buy a tin of fibreglass body filler and a rattle can of primer paint, both were bought at Canadian Tire. That amounted to less than $30. Of course, we melted a few feet of welding wire and used a little electricity, too. †