I have to fix a patch on the sills of my car, the only sheet Ive been able to get is 1mm, i have to weld this to thinner material on the sill. will this be a problem? any ideas?
Should be fine ,if you patch over the top of the sill it will require more filler ......if you butt weld the patch in make sure the front face line up with the existing sill...either way as long as your patch material is at least as thick as the original no problems
And favour the new thicker (!) metal slightly, because the old thinner original stuff will blow through first.
I usually cut out a bit more of the old sill than I think I need to and examine it carefully at the back for pitting and less obvious rust damage. Such corrosion thins the metal to half it's original thickness, and you can end up trying to weld tinfoil. Sometimes you can get away with a lap joint on this sort of nightmare where a (proper) butt weld is impossible.
Years ago we used to weave rust with gas, but you can't do that with a mig.