i use parks lacquer sanding sealer. but that's only because that's the brand my local hardware store carries. any hardware store with a paint department or, of course, a paint store will have some. get a relatively inexpensive brush and some lacquer thinner for clean ups. a quart of this stuff goes a long way.
just brush on a thick coat front, sides and back with that little extra that i was talking about on the edges of the material. let it dry for a few hours, sand it down and give it another coat. continue until you can't feel the texture of the material and have a nice smooth surface all around and then do your sunburst and top coats.