Welcome to OCC!
If the tank is rusty or has "varnish" type deposits,
Lacquer thinner dissolves it along with a hand full of small nuts thrown in or small rocks from the driveway.
Then shake it vigorously till you get tired, or it is clean.
I will not, apply a "coating"
if it leaks, I solder it, if rusty, leave it alone after it is cleaned thoroughly.
if it is kept with fuel in it, it will not rust.
But that is my experiences, your results may vary.
