A sheet of beadboard has finished seams designed to fit snugly together. When properly installed, you can't even tell a seam is there. The problem with seams arises when you have to cut a piece of beadboard to fit, then you have what is called a butt seam. When you properly cover the butt seams in beadboard, just like the finished seams, you will not even notice it is there.
Things You'll Need
220 grit sandpaper
Fill the seam joint with wood putty. Using your putty knife, smooth wood putty over the beadboard seam until it is even with both sides of the adjoining pieces of beadboard. Let the putty dry.
Sand the covered joint with 220 grit sandpaper.
Wipe down the seam with tack cloth to remove the dust created by the sanding.
Refill the joint until the dried putty is even with the adjoining pieces of beadboard. You may have to do this a couple of times because wood putty shrinks as it dries.
Always allow at least 24 hours for the wood putty to dry and cure. If you do not, the putty will come off in chunks and you will have to start over from the beginning.
Always prime the putty at the joint. You have to prime wood putty before painting it or the final paint color at the covered seam will end up darker than the rest of the beadboarded wall.
Never skip covering the seam at least twice. If you skip covering the seam twice you end up with an unsightly and noticeable divot.