If you get it to flat it will always hold water, I put resin on the board and the hatch, put them together then laid my matting, before it kicked I laid the whole thing down(with the board side up) on the concrete and put 1/4 inch shims between the ground and the edge of the cover, then put wax paper on top of the wet glass and laid a tire and wheel in the middle. The end result is that the cover is about 1/4 inch high in the middle so it doesent hold any water when I wash the boat off. also being a little concave makes it stronger.