Um, no...you don't want to try to spread silicone aquarium sealant on flat surfaces...no, no, no...it's used in the corners, just like in actual aquariums. That's all- don't even go there.
Look into the pond sealants, perhaps, for what you're thinking- & again, I've never used them, but someone else here mentioned them a while back, if you can search that out. I don't remember who or how they used it, only that I became aware of such products att & thought they looked "interesting". (I enjoy hardware stores & thinking 'outside the box'.)