Every grain filler I've used, including Aqua Coat, will fill most of the oak grain holes and cracks in one coat, but it won't hide the grain pattern, if that makes sense. Multiple coats of grain filler would have to be applied to make the grain pattern flush with the surface if that's what you're trying to do. What I do is apply one coat of grain filler, sand, and apply two coats of BIN primer. The BIN primer, combined with the grain filler, fills the grain really well to make the cracks go away. This reduces the grainy look so it doesn't look as much like painted oak, but if you want to make the surface totally smooth, with no traces of the oak pattern, multiple coats of filler will be necessary.