First, I cut the ends of the molding at 45 degrees (same as a miter cut) to join them. This helps align the two pieces, since one lays on top of the other, and provides a large glue surface. I glues the end together and nail through one piece into the other.
Once the molding is in place, I rub wood filler into and small gaps. If the walls or ceiling are not straight, the molding won't typically fit exactly right, and the gaps can get larger. I had a couple cases of this in the theater, so I needed to do some sanding to smooth things out.
To sand the moldings, I created the profile block shown here:
Here is how I did it:
- I took a shorter piece of molding, set it face up on a piece of news paper, and laid plastic wrap on it.
- Mixed up some Fix-It-All (same consistency as sheetrock mud) and put a on 3/4" layer, letting it run a bit over the sides. This stuff dries fast, doesn't shrink, and is pretty hard. Click on the image at the right for more info.
- I took a small piece of scrap plywood and pressed it down onto the Fix-It-All. This forced it solidly into the profile of the molding, and formed the support for the sanding block.
- I let the block harden for about an hour
- I broke the excess off and trimmed the block square on my table saw.
- I wrapped 100 grit sandpaper around this to sand the molding.
I may try to make a couple more of these with cement mortar, which would be really hard. I have read that some people use Bondo as well.
Here is a picture of a joint in the crown molding, after I sanded it. The best way to do this was to tip the block slightly up and sand the upper profile, then tip it down and do the lower one. I used the square edge of the block to sand the flats.
This photo shows the same molding with one coat of paint on it - the seam has virtually disappeared. I applied this coat as a "primer". Once the final coat goes on, it should be virtually impossible to see the seam. I sanded about 75 feet of crown, with a number of seams, and they all worked well.
I made a couple of blocks in case the first one broke (one is none), but it worked fine with no signs of wear. I am going to coat the surfaces with poly to help harden them further.