If the painter was comparing two different solid wires like ER70-S6 vs ER70-S2 and they were dealing with little silicon islands, the wire brushing issue might make sense, but per OP it's FCAW wire. Doesn't matter if it's a gas shielded or self shielded wire, there's still going to be a layer of slag if it's a flux core consumable. Since either the welder or the painter is going to have clean clean off that slag before painting, I doubt a minor difference in silicon content of the wire is going to make any noticeable difference.

For welding 10 gauge carbon or stainless I think a hard wire (GMAW) weld would be probably be the most ideal and cost effective process, but... any decent welder should be able to handle 1/8" sheet metal using .035 71M (or even 3/32 7018 in a pinch) all day long without having problems with wire stubbing or burning through. 8 and 10 gauge measure 9/64 and 11/64 nominal - both are thicker than 1/8" and comparatively easier to weld, so the problem shouldn't be due to it being a flux core wire.

Your guess is as good as mine what's up, because I get the feeling that nobody working at this company really knows what they are doing. That's a management problem, not a welding problem so it's outside my lane.