I have built a set of free standing pantries. Each is about 3' wide, 2 feet deep and 6' tall made out of a combination of 3/4" maple plywood and maple hardwood. I have them all stained and they look good, but I am concerned about applying the polyurethane on the interior and exterior vertical surfaces. Applying it on flat surfaces is enough of a hassle and wee bit of a chore to do it without creating bubbles. Looking for suggestions as to specifics for applying poly on a vertical surface. I'd rather not lay the things down and run the risk of scratches.
Preventing runs is as easy as following one simple approach: Start heavy, finish light.
What that means is that you immerse your brush to maybe inch of depth, scrape off one side on the can, and apply the remaining product load to the project in a new area adjacent to the previous work. The idea of "starting heavy" is that you apply enough product to thoroughly coat and fill the texture of the surface.
You then brush to distribute that load, working it outward until you achieve a uniform film that's blended with the previous area. This will not run. Finish with long strokes parallel to the grain. Do not work the finish so long that it begins to dry and get tacky.
With a bit of practice you'll be able to apply a uniform, drip-free coat to surfaces at any angle.
i think the easiest way to do what you want is twofold. first, apply the coating with a spray gun, second, change to a lacquer clearcoat. this is pretty standard for all furniture and casework. lacquer goes on really thin and dries really fast. then sand it with 400 grit. reapply as necessary and repeat until you get the build thickness you desire. lacquer is far harder than polyurethanes, but not as flexible. lacquers are for indoors, urethanes for outdoors. with urethane, it will take days to do 3 or four coats (minimum for a quality furniture clearcoat). with lacquer you can do it in a day. just remember to use a good niosh respirator and filter for VOC's