In this excerpt from a Finish Carpentry Expert Forum thread entitled "Big Crown", a veteran contributor answers questions about the best way to keep miters tight in large crown molding profiles.

09-27-2012, 07:10 AM

Throughout the last five years or so, I have installed larger and larger crown much more frequently. The crown is also getting thinner and increasingly more detailed. Do you guys still take the time to cope the big stuff? I find that the joints do not look nearly as good as a smaller coped piece. The MDF is thin and hard to get a clean cut on without poking through to the visible side. — cfullen

09-28-2012, 09:23 PM

I’ve never coped MDF crown for that very reason — the edge just won’t stay together all that well. I’m in So Cal, so not huge [moisture] changes throughout the year. I do glue it, though — CA glue [cyanoacrylate adhesive] on certain applications and splices, and Titebond Liquid Hide Glue on inside and outside miters. It’s got plenty of open time so I can go back to my corners and fine-tune them before the glue starts to grab and set up.

I just did 2,000 l.f. of 6.5" with as much detail as possible to put on a crown profile and it painted out great. I’ve never had a callback because of open miters since this stuff came out, so it works for me. — M Smith, Whittier, Calif.

09-29-2012, 09:18 AM

Just curious, Smitty, but if and when you get a callback to that job, or one similar, re: a miter opening, how would you fix it? Redo the entire piece? Bang in a couple of more nails? Caulk it? — rogerg, Vancouver Island, B.C., Canada

09-30-2012, 12:45 AM

We’re talking about paint-grade MDF here. My joints are always “light tight,” so, if anything, there might be a slight gap, which would require some white painter’s caulk and a little clear waterborne poly to lock it in so it won’t attract dirt like a magnet. Even though I’ve never had a callback, it doesn’t mean it hasn’t happened … if it ever has, the client probably thought it was a painter thing and called him. — M Smith

09-30-2012, 09:47 AM

Thanks, Smitty. What on earth is “waterborne poly”? I’ve got a few guesses, but … — rogerg

09-30-2012, 02:02 PM

“Waterborne poly” = any water-based clear finish, without any amber tone added to mimic an oil finish. A small artist brush would work great for the most concise control and placement. — M Smith

10-05-2012, 09:41 PM

Thanks for the input. I will likely try the liquid hide glue on the next project. We use CA glue on most outside corners. Love that stuff. On some super small rooms, we glue all four pieces together on the floor, then hoist it up. — cfullen