What metal is TA22 body shell made of. Sh-- to weld new panels to? Ah

[ToyTA22]

I'm replacing the outer sill on my TA22 and am having a s--t of a time getting weld to stick to original body metal.
I have done usual things such as cleaning weld area with prepsol before grinding paint off, but when trying to weld a black soot oozes out of metal and stops weld from bonding when plug or stitch welding.
The new sill is NOS original. Weld sticks fine to it. when repairing inner sill had same trouble, weld not sticking to original metal but zinc anneal repair pieces, i think, sticks to fine.
I have built a Escort for rally prep from ground up, which requires a lot of welding with no problems, also repaired old Falcon and Holden panels no problem.
I have no idea if these were made from recycled scrap, my friend used to say early Jap cars were recycled Coke cans because they were hard to weld.
Don't worry I love Toyotas, especially the early Celicas and Corollas so won't be putting a match to it yet

[allencr]

Magnetic?
Is it close to a leaded seam, doubt any repair was done with lead?

[masterofsinanju]

Has someone installed the old school black/bitumen style rust proofing in nearby areas?

[ToyTA22]

Thanks for reply. Yes it is magnetic. No there's no leaded seams. I removed old outer sill, which was rusted badly, and repaired front and rear of inner sill panel before treating with penetrol then painting in red primer. I cleaned with prepsol before grinding off paint and penetrol then cleaned again where plug welds will go. It only spits and gives off soot on original metal, not where repairs at front and rear where done, when doing plug welds along bottom and welding in door jam of new outer sill.
I found same problem when welding new floors in, had to drill out a number of plug welds and redo which was annoying. That's why I asked what sort of metal is it.
I did do four years specialized welding at tafe and used to make galvanized tanks.
This has me stuck.

[ToyTA22]

Has someone installed the old school black/bitumen style rust proofing in nearby areas?

Thanks for reply. No there isn't any bitumen type rustproofing on panel.
As in my reply to allencr this has me stumped why this metal is so difficult to get weld to bond. The original NOS sill panel is easy to weld to, just the original metal is difficult.

[YeoldeTA22]

Never heard of anybody having these problems before, you should put up some pics of the area and the black stuff that is coming out, would make it much easier for people to work out whats going on.