how to weld rusty sheetmetal

I need to cut out bad sections and weld in newer, cleaner metal. the fender skins rusted thru where dirt got between the skins and the fender frames. Is grinding away the rust at the edges of the cuts enough, or should I treat the metal with Ospho and then grind the edges?
My welds are to high and I want them to be lower to reduce the grinding effort. What do I need to change to do a better job. Move faster seems obvious. My wire feed welder had a hole in it and I wasn't getting enough gas to the metal surface. Now that I have replaced it, I hope I can do a better job of welding.
TY
SDE

I would cut out the bad, cut it far enough away that the metal is up to full thickness, away from the rust.

Use a 4" grinder with a cut off wheel, make straight, square cuts. Polish away any surface rust or paint. Then make a cardboard pattern, transfer to the new piece of steel.

Turn the welder down low enough so not to blow through, clamp the piece in place and tack. Make any corrections on position, then start stitching it in. Go place to place to avoid heat distortion, playing connect the dots with the welds until it's all one weld. If it tries to burn through or there are wide gaps, you can use a back up piece of aluminum, brass, copper as a heat sink. The weld will puddle against it without sticking.

Then grind it down, fix any holes or low spots. I like to use the flap disc type grinding wheels. They are more expensive, but give a much better finish.

These days you can buy NEW reproduction fenders. for what they cost,I wouldn't waste my time if the fenders were rusted really bad.

These fenders have two ribs and I wish to retain that feature. So I will need to repair them.
TY

OK,that makes sence...

Thank you for the advise. I will try to do the square cuts on the next section that I need to repair. It does sound like it will make the job easier. I thought that the stitch method might have been the way to weld it also.
TY
SDE

I would not use an arc welder if at all possible.

(quoted from post at 05:41:01 09/18/18) I need to cut out bad sections and weld in newer, cleaner metal. the fender skins rusted thru where dirt got between the skins and the fender frames. Is grinding away the rust at the edges of the cuts enough, or should I treat the metal with Ospho and then grind the edges?
My welds are to high and I want them to be lower to reduce the grinding effort. What do I need to change to do a better job. Move faster seems obvious. My wire feed welder had a hole in it and I wasn't getting enough gas to the metal surface. Now that I have replaced it, I hope I can do a better job of welding.
TY
SDE

Click to expand...

Don't treat. Get to clean metal. If you can do that by grinding, then grind, but more likely cut our sections and leave a clean edge for welding. Make your patches carefully to fit in the cut section.

Once they fit right, mix up a small batch of JB Weld and fix the patches in place with a dab of the JB Weld in the corners. This is just to hold in place, it is not structural.

Turn down the gas pressure on your wire feed, and turn down the amperage too. It sounds like you are blowing some of your material away with the gas. Just enough gas pressure to keep the weld spot flushed with the inert gas. Just enough amperage to be able to strike and keep the arc.

Get close to the work and support your welding arm/wrist with a brace. Do small sections at a time so as to not warp the patch or fender metal by getting it too hot for too long. You should have a bunch of small dabs of weld spots all the way around the patch edge. Knock the JB Weld off. Complete the fill in job with the welder, but allow it to cool often so it doesn't warp.

Once you have the entire edge line welded, go back with the grinder and grind it fairly smooth. Prime, and paint.

You cannot weld rust, IMHO you are much better off to find replacement parts, should be a ton out there.

I did some 8N fenders with the same problem. First get them sand blasted. They need to be clean. Cut out the bad stuff. Set your welder to just about as low as it will go with the finest wire you can run. Get some copper chillers to back up the metal where you weld. Clamp the chillers right up tight to welding area. The chillers will allow you to weld the metal but carry away the excess heat. It takes some practice, but it will work. I got the chillers from Eastwood Restoration supply. They are annealed so they are also flexible to help match curves. You will also need to learn how to shrink the metal repair areas. My patched areas were oil canning real bad so I had to shrink most of those areas. I learned a lot about making this type of repair. Just take your time and enjoy the learning experience, I did. I have a beautiful restored 8N to show for it.

OTJ

This was a great question/topic, one I wish I could be better at.

Regarding shrinking metal, could someone give a topic on how to do that?

Thanks.

Paul

Good advice here. Just don't be in a hurry. One thing good about the fenders they are good thick material. Stan

There are lots of videos on line that show how to do it. I just use a small brazing tip and heat a little spot at a time. As it starts to show some color quench it with a wet rag or a spray bottle of water. Do little spots at a time. If it is a big spot work in a circle gradually working your way to the middle. Always start small...

OTJ

I have some magnets that I had hoped to be able to use, to hold the patches in place. I was going to try welding one of them today, but I did not get to it. Maybe tomorrow.
TY
SDE