I want to repaint our steel entry doors, which presumably were
pre-primed 30 years or so ago. Where the later layers have peeled off,
the earlier layers still seem to be in good shape, and there is no sign
of rust.

Is there any chemical stripper that would be very effective with those
more recent (latex?) layers but have little effect on the original
primer. I'd rather not strip them down to bare metal and start over if
it can be avoided.