I recently saw an "auto body repair guy" weld into grinded down rust. I was wondering if I could simply weld into filling in and around the rust spot with new metal then grind it smooth and bondo it?? What I am asking is - if I do the weld in and around rather than cut will it last my lifetime?? I don't have any offspring (at least none I am aware of) so I don't really care if it goes on decade after decade.
I thought the thing to do was to cut it out then weld in new metal. I would much rather do the weld in and around trick.
Here is nearly 30 years of rust I am trying to repair, please help!

 

I don`t do body work, but the best way to repair that rust would be to cut it out and then cut a new piece of metal to fill it in. To weld it you would use small tack welds all the way around the repair, stopping every so often to let the metal cool so it doesn`t warp.

 

We fiberglsass it from the inside out.

 

The problem with cutting and welding is the location. This rust problem is in theee worst spot. It is inside the door well. I have cut and welded from hell to breakfast on this truck. It's getting there but GDamn it is frustrating. I was hoping to cut a few corners with no major catastrophe outcome

 

Im sure there are ways to cut corners, but to keep the strength of the cab the correct way to fix it is by cutting and patching. You could fiberglass over that section but the rust will still be behind the fiberglass eating away at the cab.

 

What about rust within new welds, new metal??

 

I would just cut out as much rusted metal as you can get to and then replace with new metal. once you cut out the rusted metal i would see if there is something that you can use to treat the metal with that will kill the rust.

 

Cutting it out is the best way. However, a big issue is going to be treating the patch and welds on both sides so the bare metal won't rust from the inside out. Like Neil Young said 'Rust never sleeps' and it will come back, how fast has to do with treating the metal after you patch it and how much salt the truck sees. BTW, I was an AutoBody tech for a few years and have seen my share of mayland rust vehicles.
Another problem can be flash rust when bare metal is left un treated too long from humidity in the air. Sorry to tell you that short cuts will not yeild rust free, but treating it with something like POR15 will slow it's progress down.
Good luck,
RB

 

I ended up cutting it out and welding in new metal. WHAT A TOTAL PAIN IN THE A$$.
It is going to take me a few days to get the bondo to shape the same as the original contour of the door.
For hell sakes!

 

Use thin coats o mud/bondo. It's in a door sill so I wouldn't kill yourself over making it. Perfect. If it's a tricky area use a cheese grater to knock the mud down while it's soft but not wet. This also helps if you aren't efficient at spreading mud. Sand and then topcoat. If you are using bondo brand(local parts store mud sucks honestly) try to get a MarGlass product it's easy to find from a body shop suppllier and sands much nicer. I have had the cheap bondo peel off the metal before.