Master4g
This is a product that may fix your problem:
http://www.mcnett.com/Tent-Sure-Tent-Floor-Sealant-P169.aspx
scrub off the flaky bits before use.
It may or may not work
it may or may not cause cancer/world peace an outbreak of common sense and or other nasty events...
(IE : no experience with it but for $10 is worth a shot since your fly is cactus as it is...)
Franco