Carpentry and Woodworking - Wood movement with long runs of molding

Hi,

In the past I have spliced crown molding in place (in place meaning the first section was already installed because the entire spliced length was too long or too heavy to handle) by cutting a bevel at about 20 deg, and running the biscuit joiner into the end while laying on a flat surface. I used the biscuit joiner at the #20 setting but of course you won't get full penetration because of the bevel. Then I used yellow glue and a #10 biscuit. With a little bondo type filler and some sanding I could make a near perfect joint. However, in the case of the crown, the joint did show somewhat after a year or so, I think mostly because I was unable to prime the back side after the splice since it was installed/spliced in place. It didn't crack or pull apart but you can feel and see the joint to some extent. I have heard that even factory finger joints may show through after a while if the back sides of moldings are not well primed.

Also, I just noticed that on the 25 ft crown run, the caulk joint at the coped end of the run also cracked. It may be too much to ask of a 25 ft run. Is it better in a long run to let the spliced joint float (ie, no glue, no biscuit, just bevel cut, nail over blocking and use caulk) so the splice acts as an expansion joint of sorts? I took all possible precautions I could think of, including 2 coats of oil based primer on the back side and I think I even sealed the coped end. I have seen recommendations not to nail too close to the coped end but I've never understood why this would help.

Let me add however, that when I installed the crown in this room, after caulking I wiped down the crown with mineral spirits before final oil based paint. However, over the next few days the caulk cracked in several places even on the side of the crown. Now that I know more about paint and adhesive chemistry I know this was really stupid. Although I fixed this problem over a year ago, it may have had a residual effect. It's not a real big deal because as it turns out I have to repaint anyhow (color change).

Appreciate some input from the experts on how they handle long runs however. I use MDF whenever possible because IMO it has many advantages but is is very difficult to handle on long runs because of the weight and my lack of helpers.

Thanks

Don't have a lot of good anwers for you but as you know large crown moulding is a bear to put up and to keep caulked. I think it has to do in part from lack of solid nailing surface. MDF paints real well but I have seen it come loose alot on crown moulding [possibly from the added weight?] Due to the way crown is installed it is hard to get caulking to hold up. IMO the best caulk to use is of the polyurathane type but it is hard to find [pricey too] Second best would be a silconized acrylic caulk. Wish you luck :)

Are you cutting the bevels in the correct direction? It sounds stupid, but in some cases there is a right way and a wrong way. You should try and cut the joint so that you look "over" the cut and not "into" the cut. Sometimes it isn't possible because of the layout of the room, but if you can, it will make a big difference in the visibilty of the joint.

Also, sometimes it is better to put nailing blocks behind crown moulding. make them big enough to fit behind the moulding without effecting how it lays and then it gives you better nailing surface and more holding power. Depending on the moulding, sometimes furring strips will fit perfectly behind it.

Good Luck.

I used 2" wide triangular pieces cut from 2x4 Doug Fir and screwed into the studs for backing. Not very substantial and in fact I basically assumed that even using a pnuematic finish nailer some of them would split. Also a real pain to rip 2x4's on a bevel even with a contractors saw - keeps bogging the motor.

Furring is a good idea. Even if you had nip a corner off of the furring that would be OK. So do you pretty much use backing over the full length?

Furring is a good idea. Even if you had nip a corner off of the furring that would be OK. So do you pretty much use backing over the full length?

I have done both. The nice thing about doing the full length is you can put nails any where along the length and it doesn't matter where the joints go. With individual blocks, you have to remember or mark where they are so that you can hit them with nails.

If the walls are really wavey and or the ceiling runs off, it is nice to have the full length covered to help draw the moulding tight where it needs to be.

If you find the anwer to cracks in the caulk, let me know, lol. I notice that when caulk is applied too thin, it will crack. Not too thin, not too thick is best.

I never caulk coped ends so I don't have to worry about movement and caulk cracking.

Nailing too close to the end, you ask? That is only a warning so that you don't split the wood by nailing too close to the end. You must nail close enought to hold the end firmly, say 2 or 3 inches or so from the end so you will hit the corner stud. You want that corner joint to be tight. Splitting is more likely to occur on loose joints where the end grain is not under any compression from tight fitting joinery.

On splices, I always measure and locate the splice over a solid nailer backer, even if this means cutting off 15" of crown to get to the last wall stud. I fit and nail splices as close as possible to the the splice. If the trim is deep enough, I nail into the splice to pin the two pieces together with one nail.

I do not prime the backsides of interior trim. That is more of a concern for exterior work.

You will see your joints and splices because only you know where to look. Splices and joints are not invisible, they are merely less visible.