Hello,  I decided to keep my post about acetylene very short and
probably left out needed detail.  I was also not aware of the new
torches that are available.

It was good to be reminded about zinc poisoning.  This is another
reason to use a very soft heat when soldering galvanized sheet.
A large copper soldering iron is really the best tool.

It is only the cheap propane/air torches that are not good for
critical work.  Propane/oxygen torches are completely different.  They
can use a variety of tips and can be adjusted to a suitable flame for
careful soldering.

Cheap air torches (there were also torches that use pressurized air
and these can work well and may still be made) do not have a primary
air supply.  All the oxygen needed for combustion is introduced after
the fuel has left the metering orifice.  Because of the characteristics
of propane, this requires a high velocity jet of fuel and a large
volume of air.

Better torches do have a secondary air adjustment and can be made to
produce a soft flame but it is usually not very stable and can not be
made small for pinpoint accuracy.  Using MAPP gas in a fuel/air torch
that is designed for it can produce better results, but nothing beats
acetylene for delicate soldering.

I didn't realize that my Prest-O-Lite acetylene torch was so expensive.
I probably paid $10 for it at a flea market, 30 years ago.

Best regards,
Spencer Chase