I did my aluminum a year ago and the coal tar is still mint, no leaks.
Im not sure about your way of welding, I belive flux will leave little holes in your welds that water will get through. you need to weld with a...
If you plan on powder coating your material you cant use wd40 or a oil base product, if you do so the powder coat will fisheye
Have you looked into Cold Tar Epoxy?
I would use a band saw forsure, If you want to lube your blade use cutting wax, then wipe down your material with laquor thinner before you weld...