DO NOT coat your bacground with resin, I made a fiberglass + concrete background that was GREAT but then I decided to coat with fiberglass resin to prevent the breakdown over time and the PH swing.
When I put the resin on I put 2 coats on with a brush, and it trapped air between the resin and concrete, not bubbles but it came out as a milkey white film when under water!! i'll post some pics whne i get to my PC.
My 2 cents .. I would use a thick layer of concrete... TheDeeB used a thin layer like paint. I used a concrete mix with fiberglass threds in it, supposedly stronger. but your mixture w/ resin sounds like it accomplishes the same thing. GL!!!