Any stripper will work, unless powder coat, in which case you may need
to get aggressive. There may be stinky strippers, I can recall some from
years ago, but removal of the stripper mechanically and rinsing the bike
off should take care of the odor. OTOH your bike will begin rusting
within hours and even clear coating will not prevent this. You will almost
certainly have small rust pits present somewhere, and these will not be
passivated by clear coating. A phosphate under treatment is a good
idea and then a primer coat is a better idea. Steel just does not do well
without paint of some kind. You could blue it or parkerize it or nickel plate
the frame but this will just slow the rusting down. Entropy will out. There
may be a clear coat that will reliably stick to bare metal but clear coats
are designed to go over other paint layers as a final layer, so you will have
to rummage about for somewhat exotic coatings to find one.
A can of rustoleum primer and rustoleum hammer tone gray would run you
about $15 or so at Lowe's and you might like the result. I used the stuff on
a large metal cutting milling machine about 5yrs ago and it has held up well.