I try to get the welds to penetrate, but �the rusty steel burns through pretty easily. I'm finally learning to fine tune the heat and feed when I have a rusty area, but when I have new steel next to rusty it's tough. I know what you mean though. The fellow who is machining the engine told me to weld from the good steel to the rusty...get the arc going on the good steel and then move to the rusty areas...that works pretty good, but burn through still happens.....the right heat is the trick but it is still a learning process...
Oh, and as with so many things, prep is everything....clean steel, no paint or oil..I grind it clean and then wipe it down with denatured alcohol...
It is sooo much more fun to weld new steel to new steel....It might have been worth it to buy all the parts from the toe boards back and just build a body...
I think someone suggested doing this earlier on!!