I've got a tank made of mild steel sheet metal, and I want to
permanently attach some aluminum tubing to the bottom surface of the
tank. The tubing is for some hot fluid (200F-250F) that will be used
to heat the contents of the tank. The aluminum tubing is only 3/8"
diameter, with a wall thickness of about 0.060" (maybe 1/16"). I
estimate that the sheet steel that I'm attaching it to is about 16
guage, maybe 18 guage (0.050" to 0.060").

What's the best way to do this and the best tools to use for the job
for a DIY type person like myself. I'm not a professional, but I do
have some experience welding and soldering. I have both an
oxy-acetylene torch setup and a cheap Propane torch that mounts on a
small hand held propane cylinder.

I've done a bit of reading and research, and I think I should probably
use some Cadmium/Zinc solder, probably 30% or 40% Zinc. I'm not sure
exactly what flux to use on the steel, or on the aluminum.

I'm kind of guessing that the propane torch would be a better choice
for this job because I'll need more general heating of the materials
and indirect heat. I've got some extra material of both types to
practice on as well, so I won't start out ruining my main project.

I'm just asking for advice or confirmation that I'm on the right track
here. Also, would a normal welding supply place have the appropriate
solder? What kinds of flux would I need for this? I plan on calling
them tomorrow and asking, so I guess I'll find out.

One resource I checked said not to use flux on the aluminum, but to
apply the solder with steel wool to scrape away the surface oxidation
on the aluminum. The procedure described was to heat the aluminum with
the torch, drop a few drops of melted solder on the aluminum, then use
steel wool to work the surface of the aluminum while continuing to
apply heat to maintain the temperature slightly above the melting
point of the solder. As the oxides are scraped clean, the solder will
wet and tin the aluminum. I'm probably going to be soldering down 12
to 18 feet of this tubing, so that sounds like a lot of work with the
steel wool. Would there be a better method using some kind of
chemical treatment or flux to remove the surface oxides from the
aluminum?

I plan to tin both surfaces to be joined with the solder, then sweat
them together and apply additional solder to fill any gaps and build
up a little under the tubing. I want as much heat transfer as possible
from the aluminum to the tank in the finished product. I'm assuming
that I'll need 30% to 40% zinc in order to have some decent paste
range to work the extra solder into the gaps and build up around the
bottom half of the tubing.

Also, this surface will be exposed to weather (after painting), so
corrosion resitance is fairly important, as well as strength.

So, am I on the right track? Or am I about to make a huge mistake?