Allen,
I am a professional only in that I sell my work to pay for my addiction, and I do it "full time" to keep me off the golf course (I am retired). But my spray booth is outside, in the area between the garage door and the cars. May not work every day in Long Island, but I know people who spray USL indoors.
The purpose of a filler is to... fill the grain and give a smooth surface. On mahogany, I spray a coat of USL, then sand it as if I had used a sanding sealer or specialized filler... down to the wood (yes I use coarse sandpaper - 400 grit) on a power ROS. It may take a second filler coat before the holes in the grain are filled level with the surface, but that is similar to using a sealer or filler... and much faster/easier. Then I keep spraying with the same stuff, allowing it to dry for a half hour between coats. Ideally you should let it dry for hours or a day before the final finishing, but I have completed a piece in hours from the start of filling to ready to deliver.
I consider USL ideal for beginners since it is so simple and forgiving. The big feature it has over Polycrylic (which I have used) is "full burn in" meaning that each new coat blends into previous coats, so when you sand or rub out, there is no challenge. If you sand through the finish, just put more on - no witness lines.
I have USL on my own kitchen table, and have used it on tables that I have sold (the customers seem happy). I have also used the conversion varnish that Sam used on that table, but I don't recommend it for a beginner.
I have a couple web pages on these finishing techniques at
www.solowoodworker.com/wood/finish.html (I think I am allowed to post a link to my own site on this forum, but if not, I hope the moderators will just remove this paragraph, not the whole post.)
