Re: good idea or bad one
I used the West Systems epoxy resin and hardener. They make a good product but they're proud of it too. The expanding foam can cost you a few dollars. Glass mat and cloth comes next then primer, color coat and clear coat. They all add up and you should consider your labor which can be extensive. Then there's sanddpaper, cheese graters for forming the foam and all those other little things people don't consider when fabricating a part.
Just thought you should know it's not the least expensive way to go. Just for example I bought a new steel tank from TC Choppers for a bike that cost less than what I put into the red white and blue Captian America tank.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"