After drying a full 24 hours, if there are noticable air bubbles, just use fine sand paper and gently, very gently rub over the air bubbles not to push down but skim over them. Wipe off and put another coat of seal on them and they should turn out perfect. Regular paper pushes air bubbles out as the sealant/poly soaks in. It takes a little more time, but a lot easier and less expensive than photo paper.
Try different techniques, moving slowly with your brush on some sample pieces until you've perfected it. Oh yes, and don't let dry in a cold room, it should be room temperature. I found a spot in my house that once they're ready to dry, I leave them in that spot for a couple days. You also want to make sure where ever you set them to dry, that it is a level surface. Otherwise, you'll have more sealant/poly on one side than the other and it is noticable. Take my word for it. Once all dry, put your coaster legs on the back and done. You can also get some of the pic stands if you don't want to give them as coasters. Or make a wall haning, the ideas are endless.
Trial and error, I've been going thru it without having learned it first. I just wanted to make them and had an idea in my head. I'm sure there are many ways to make these, my main concern was making usable coasters. I'm going to be experimenting with different sealants as well and will let you know how it goes. You can do anything if you set your mind to it!