This is a custom door built to a client's specs. It's a narrow door, only 31" wide, and was to hang on contemporary barn-door-style rollers. The client wanted clear, vertical-grain fir to match her existing refinished vintage floors, and the door was to have full-thickness, vertical-plank inset panels.
Since the inset panels had to float in the door as any inset panels must, I built them as shown in the photo and glued each panel up solid. Then each of those panels floats on four splines in the door frame and is centered with "Space Balls," small neoprene balls manufactured just for this purpose. They allow the panels to float, yet keep them centered.
Since this door was to hang from its top and would never be subjected to the stresses placed upon a hinged door, I just used four biscuits at each joint of the frame. I've done this several times on sliding/rolling doors and have never had a problem, even when I was building doors in Hawaii's humidity.
The client wanted the door to have a vintage look, so I didn't plane the 2x stock, but just sanded it with an orbital sander prior to stain and finish. This left a less-than-perfect irregularity and a few lumberyard bruises here and there that darkened nicely with the stain.
I used Minwax Polycrylic satin for the finish, spraying on four coats. The finish is nearly flat, just as requested. I've had really good luck with Polycrylic for interior doors and trim, and even some furniture. I brush it on small projects and spray for something like this where it would be difficult to keep a wet edge while getting into all the nooks and crannies.
My "rotisserie" for finishing doors works like a charm! The trick is to drill the holes slightly off-center so the door is a bit heavier on one side. Then a 1×2 gets screwed into one corner on the heavy side and hangs down to catch the weight and keep the door firmly horizontal. I use 18" long pieces of 3/8" steel rod and the holes in the door are about 4" deep.
This project started out as a request for the art-glass panel that is in the door, but when the client asked if I could also build the door, the adventure began!