Is the metal sound with joint leaks or corroded and decomposing? I have a
galvanized roof system I designed and after 20 some years had to repair
it. I used a product that adheared to the metal after putting on an
adhesive coat. The new was thick roof compound with reflective aluminum.
It came in rolls. I had thought of the roll on roof coating compound, but
when I realized my metal was almost non existant in some places, I opted
for the roll material which cold bridge small gaps and gave a new layer of
metal (aluminum) to the whole surface. It cost several hundred dollars to
do my 24 x 24 garage roof, but the actual area covered was about twice
that as I lined the 2 x 10 joists/rafters and then put a new deck down on
top. I was just trying to locate where I got it so I can buy some more to
waterproof under another deck. It seems very good. This isn't the answer
to the question you asked, but I think it depends upon how sound your
metal is. If it's sound then follow the label on the can for coverage. I'd
pay special attention to where it leaks and why. If there is a seam with a
crack or some structural reason that encourages a leak I'd try to figure
out how to fix it before coating. Good luck. I don't think this was too
helpful, but sometimes others ask helpful questions. Perhaps I did.

_______
KaCe