Is RG45 the most generic/general purpose fill rod that can be used on
boring low-carbon steels like A36 or rebar?

I'd be using oxy-acetylene. Just want something real plain for patching up
what's basically scrap steel. I last used ER70s on real thin wire and
sheets, but that was a while ago.

Any specific real-life reasons to pick one over the other for non-critical
uses?