I'd try for an odd-ball vehicular propane tank, much heavier wall--try
some propane
distributors , see what they have and/or will refer you to an
installation/conversion
shop. calls to auto salvage yards might turn up something 20-30 gals.
The little
20-40Lb tanks are too thin-walled--rust out quick--put water drain in
bottom!!

"Jason D." wrote:

Looking to use propane tank either two 20lb or one 40lb tanks as air
tanks?

I'll get new empty tanks FYI if this is doable. I hear they're
supposed to hold up to 300 PSI test as part of requirement as propane
tanks?

Cheers,

Wizard