My 240 is once again suffering from severe floor pan rot after a not-so-good patch job from yours-truly. I'm debating on which method to fix this - steel and welding, or aluminum and screws.

This vendor sells steel floor pans for the 240:
http://www.rustrepair.com/PANELS/VOLVO-67-BODY-PANELS.HTM
I did buy some wheel arches from them, but I'm worried about the thickness of the steel and whether or not it would work well enough. Anyway, my plan "A" is to have someone weld these panels in, then I coat the hell out of them with POR-15. But I don't want to go through the expense and trouble if the damn thing will rust and rot away after a few years. I've used POR-15 before and have had some success with it.

Plan "B" is to buy some thick aluminum sheets and bolt or rivet them to the frame (since I assume you can't weld steel and aluminum together) and fill the seams with epoxy or POR-15 Putty.

What do you guys think?

two different metals can suffer from an electrical reaction, destroying one of them.

On fire trucks they used sheets of Mylar or some rubbery material, and they still can suffer corrosion.

Now, they do not put different metals together.

Steel panels and compatible screws would be a good choice.

They now have glue that they use extensively in cars - some really high end cars are now glued together.

You cut out the part and add an offset flange then cut your patch to fit the flange. - see

http://3mcollision.com/products/fillers-and-glaze/smc-repair/3m-panel-bonding-adhesive-08116.html

As long as you have good clean steel to weld to and prep the surface correctly, I would have the guy mig weld the new floor pans in place. Flux Core (simplest process) will do well with a series of short fillet welds.

You can use a seam sealer between the new and old panels with the new being on the outside and a 2 inch wide lap joint, if possible.

Then POR the inside/outside and should work.

The key is to cut out all the rot and get to clean metal, use POR Metal Prep and make sure the weld area is clean. You can pre paint within 1 1/2 inches of the weld area without worryiong about damage and can repaint after if needed.

I have used the Lincoln 135 Welder sold at Home Despot to do similar sheet metal welding. just do short, small welds to keep the heat down.

About how much would I pay for someone to do the welding for me?

Depends on how mush demo is involved. If you do the demo and prep it can save you $$ but first find a person to weld and ask what they want $$ wise and then ask if they will be ok with you doing prep. Do what they say to the level they want it done and that should result in an acceptable repair. As to procedure (where to have lap joints, how to seal between layers, etc)that is what you need to work out with them. also be aware that in most (99%) of this type of repair there is NO warranty, so the more time/care you put into prep and post work, the better chance of a lasting repair.

In my youth, I riveted aluminum to the wheel wells of a 144 to repair some rust. Then Bondoed over them.

It lasted a few years, but it seemed like that set up some kind of electrolysis because the aluminum then started to corrode faster than the surrounding original panels. Especially where the aluminum was in contact with the existing body panel. Weird.
I guess my takeaway was to only used steel on steel or aluminum on aluminum. But it also depends on what you expect to get out of the repair.

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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig