I've done 4 planes with 21st Century Fabric and done nothing to seal or treat the surfaces before hand other than 400 to 600 grit paper and a wood pecker. I've had no problems with fuel intrussion.

Do it cliffords way when you want that perfect finish with paint or film. 21st Century fabric is a bit heavy to take advantage of a super fine finish.

Opps, everything I said applies to 21st Century PAINTED fabric. If your using the unpainted fabric it's cliffords way all the way.

I never tried minnflyers way.