I used Minwax Spar Urethane semi-gloss and am pleased with the gloss level. I, too, had specks in the finish. They looked like fuzz to me. I tried different brushes including Purdy and even a foam brush. I followed the suggestions available on Mar-K's website (
http://www.mar-k.com/installation_hints.aspx). I stirred, applied quite liberally, brushed in one direction, made sure to get the urethane in all the pre-drilled holes, and ran the brush lightly from one end of the board to the other after completing each side. I then used a lint free cloth to wipe under the board to remove any "drool". I did the bottom and sides of the boards first and sanded between coats without a sanding block (only used 220 grit), in a location separate from the room where I applied the urethane, just enough to get rid of the specks, and tack clothed to remove dust. I let the boards dry 6-8 hours minimum then flipped them over to do the top. I did this for 5 coats. I still had some specks in the finish coat (not sanded) but they are minimal and will likely only be seen by me. Note that I did only one coat on the bottom then one coat on the top. In other words I did not apply 5 coats to the bottom side before doing the top side.
The result: