Thanks so much for your help so far. I prepped the wood with weather resistant primer. One purpose for the sealer is not only to protect the wood under everything, but also to protect the paint from the sun; the sealer is UV resistant.
After talking with some women that use the same sealer on the same project, I have applied a coat, undiluted, with a high density foam roller. There are some visible, though very small, bubbles now that it is dry. I tried sanding them with a 0000 steel wool, but it's not seeming to smooth them out. Does this mean that I put too much of the sealer on, or that conditions were not ideal? I used a stir stick and did not shake the can before applying.