I'm addressing rust issues and came upon an problem that I'm unsure about.

The top surface of the longtudinal, under the (removed) battery tray, had a lot of surface rust but is solid.

After light sanding and wire brushing I treated the surface with SEM Rust-seal (39308). This is a rust "converter" that turns the rust into a permanent black coating. The product works great and as advertised, however I think it leads to the problem I'm having.

I want to weld an additional coverplate over this area and fabricated one using 18 gauge plate steel and drilled 1/4" holes in it for welding. A light coat of 3M weld-through was sprayed on both the long and metal plate.

When I try to mig weld up the holes, attaching the plate to the long, I get a great looking weld that doesn't attach to the metal of the "long" at all!!!

I even tried scratching off any form of paint (in the holes) with a dental pick.

So, two things come to mind.

It may be that the SEM is preventing the weld.

It could also be that 1/4" holes are too small.

Most of the SEM coating can be removed with Aircraft paint stripper (dang, what a messy job)

Wes Vann

Take a white metal marking pen and mark through your plates spot weld holes onto the longitudinal. Pop the plate off. Then take a small grinder or even a dremel and sand down the white marks on the longitudinal to bare metal. Now you'll be welding a clean surface. Likely a lot less mess than stripper.

1/4" holes are too small. Welding through a hole is a PITA. Gotta make them big enough to hit the base metal and melt it. Try 1/2"

Don't be afraid to sand off the converted rust. The Rust-Seal is a great way to convert all off the nooks and crannies. If you sand to bare metal afterwards, you're not losing anything.

I can't say I told you so, I didn't. For future reference, it's best to do the welding before the rust treatment if possible, or at least grind down the areas where the welding will be done after the rust treatment. The only things that you can weld are bare metal and bare metal with weld through primer on it. After the metal that will be welded (in the immediate area of the beads or spots) has been ground down, the heat of the welds will destroy the rust anyway, and the weld through primer will flow back into the spots between the panels if there's an overlap as the weld cools. These effects only apply to an area about 1/4" away from the welds, max.