When I used the Citri-strip, the paint just blistered up and came off in strips, first try. Used in on a machine base, and was very happy with it. Might have been whatever that particular paint was, but it worked.
The SDS shows it to be 30% or more N-Methyl-2-pyrrolidone, somewhat nasty stuff, but seems to work.
http://www.citristrip.com/uploads/documents/QCG73801T_SDS-CS61040.26.pdf
The peel-away #7 worked well on old paint, but did not just blister it up, it took a bit more work.
Peel-away #1 is lye in a paste, and takes off 8 or more layers of paint at a time, but requires time to penetrate. Best used on flat surfaces, because it has to be covered etc to work best.
I never liked the methelene chloride stuff, it just made the paint into a nasty paste that was hard to remove completely. But I never tried it on metal, got tired of it for removing finishes on wood, and quit messing with it.