Advice needed on flood/drain

Hey everyone I have a 4x4 flood table set up with 3 week old clones rooted in rapid rooters then transplanted in 6" rockwool cubes (roots not through 6" yet) on top of couple inch layer of hydroton. I have the plastic left on blocks and these cubes seem to take days to dry out to the point i can flood again. My grow room is 75f and 46% humidity almost always and they are under a 600w metal halide atm. I am wondering if i should take the plastic off to help let them air out or before the roots come out i should change something, I know rockwool is suposed to have good air/water ratio bit they seem to stay heavy for 4 days plus, will this change as the root system fills the cubes? Any advice would be greatly appreciated i was under the inpresion i would be flooding 2 to 3 times a day but dont see how this is possible.

Thanks.