Share some wisdom... I am soon to be installing a 6+" crown molding in a large room where the 9' high ceiling (which was feframed/blueboard/plaster a couple years ago) has a really poor plaster job with lumps and bumps, and nothing is straight, flat, or level. The wall framing/plaster is equally dynamic! I have 10' and 12' pieces, the longest wall is 20', and the quartersawn molding is thick, heavy, bulletproof, and inflexible, having been milled from 5/4". I've installed plenty of crown, but this is intimidating. Any tricks?
(Cabinet and Millwork Installation Forum)
From contributor L:
If this works into the look that you are trying to achieve, you can do it. Wrap the room with flat stock on the walls and ceiling (where the crown is going) and have a thumbnail or a cove on the edge. Use shimming to keep everything as flat as you can. It will be more costly in materials (and labor), and the client should pay if they expect a decent job with a lousy base.
I have a taper/spackler buddy who is great with a wide knife who smoothed out the bumps in my own 1927 plastered house before I installed my crown. Find one by you?
As contributor J says, caulk. We call it Prairie Caulk - for them wide open spaces.