Rust-proofing lap weld?
 

Not diesel specific I know, but IMO this is the best section of this site. :)

I am doing some rust repair in the floors of my 240D. Part of the problem was water getting in between two layers of metal on a factory lap weld. I am replacing the rusted out metal with factory sheet metal and am keeping the factory design with the lap weld, but would like to be sure that the rust will not return in this seam. Can anybody think of a rust proofing product that will stand up to the high heats that will be occurring when the joint is spot welded back in place, or that would wick into the seam after it has been welded? I have considered pouring some oil into the area after welding, but want to explore other options to make sure that my undercoating, seam sealer and paint will still stick to the area when I apply them.

Thank you for any suggestions.

I did this job on my 74 240D 4 years ago with great results. I used 18 Gage soft steel body plate because it easy to form into the floor contours. All joints where the new meets the old good metal were pop riveted with steel rivets, not aluminium. Any rust left behind was killed with a product from an auto paint supplier. It turns the rust black and ready to paint. To water proof the in and out surfaces I chose not to use under body coat but instead, at the advice of a car restorer, I used a rain gutter sealer. The stuff is black, very sticky, long fibers and can be built up to a good thick and tough layer like what is on the bottom from the factory. It will dry hard in 3 days and can be applied to a wet surface. From Home Depot, get the trowel grade.
I had to replace all 4 foot wells.

There is no paint or rust proofing product that will hold up to the heat from welding. I think you are going to want clean metal to weld or any impurities will affect your welds.
I like the POR products, but you might get other recomendations. they have worked ok for me. just have your area clean and degreased, I use their Marine Clean. their Metal ready etches the metal for good paint adhesion. give it a couple coats and maybe use their seam sealer. I have used an acid brush to work it in.
For an under coat I bought some of that bed liner brush on stuff, dries to a tough surface. don`t remember the name of the product, but worked good.

Charlie

Por15.

There are anti-corrosion zinc primers that are designed to be used in weld seams. 3M makes one but this one is by far the best I've ever used.

http://www.levineautoparts.com/upwelprimaer.html

IMHO all seams should also be sealed with an epoxy based seam sealer.

Use a welding primer. It's designed for the job. After you have done the welding, consider using POR15. POR does have a thick gel type protectant that you can get. If your not sure then at least use a brush on seam sealer. A paint jober should be able to provide the seam sealer by 3M.

http://solutions.3m.com/wps/portal/3M/en_US/3MAutomotive/Aftermarket/Products/Product-Catalog/?PC_7_RJH9U5230ONO702LE2UNGDM644_nid=GSNKJYXPHDbe6DTWTJQPBBgl

the seam sealer it a thick paste type urethane that never hardens. it always remains flexible but skins over and can be painted. It is what Ford uses on all there cars. We did the 65 Mustang after welding sections with the seam sealer.

Dave

You could lead the seams after welding. I think the rear seams on MGs and stuff are leaded to seal them. I think old time restorers use lead to fill in rust spots and seams. The flux should clean out any rust. They also make a low or no lead (metal) seam filler. I haven't used any though. The POR-15 with a metal prep would be worth a try.
Tom

bgkast......some have mentioned the welding primer.....I have used it on some of my early restorations....its ok but you have to understand that anything on the metal including the welding primer burns off and the fumes and contaminents do effect the quality and strength of the weld.
Now if I use it I clean it or grind it off the exact area being spot welded or bead welded and then hit it with a good quality high zinc primer afterward.
To keep water from entering the seams use a 3M Seam Sealer (gray color) in caulk tube or in can and brush it on or use a putty knife.(I bought the last batch from a CarQuest) .....thats exactly how most of the factory seams are done. The stuff can be painted and or rust proofed when it dries.

Thanks for the suggestions. I am planning to coat the area with POR-15 and use a 3M seam sealer on the joint when I am finished. What about trying to wick some thinned paint into the seam after welding? I would like to do any thing I can to protect this seam, and to treat the seam on the opposite side of the car. Even the rust free parts car I cut the donor panels out of had a bit of rust beginning to form in this area.