COVERING WALL SEAMS
First, brush grade Jaxsan 600 applied with a flexible taping knife works really well. Attack the seam at a diagonal, fill well, strike off excess. Second, M-D Patch (made by GE) Stiff like putty, but works quite well. Same application as Jaxsan. Sandable.
I REALLY have to stay on students not to overapply any fillers. I try to get them to strike off excess so you can see the LAUAN :) through a light haze at the joint line. Hate sanding!
If we're talking Stage, usually dutched seams. Muslin & glue/paint. If the walls are to be painted a nice dark color, Gaff will work in a pinch. If we're talking Hollywoods, joint compound, or Gaff again.
Masking tape, IMO, is a poor choice. It looks tacky (no pun), and doesn't have the "stick" of gaff. There's nothing quite like going to a show and seeing the walls peel (been there, done that, got the swag!)
I use water putty. A thin coat to cut down on sanding later. Most of my paint jobs have at least some kind of subtle texture on them. I can count on one hand the number of scenic paint jobs that did not have at least some texture painted in. In a bind, gaff tape. It will be visible if the audience is close unless there is some painted texture. For painted texture, I use a feather duster or a textured roller.