I used 4 inch mdf baseboards which came primed on one side bought from Windsor Plywood. Having never used MDF before I just painted the side that came primed from the factory and put it up in the house. I put it up in summer. Now in winter, every inside mitred corner has open up (some as much as 1/4 inch on long runs).
In my head I'm thinking MDF is supposed to be more stable than wood (so I always heard)...so...causes...
1) I painted and primed the BB in my garage/shop and once cut brought them into the house for direct install...you MUST acclimatize it to the space. Keep it in the room to be installed for a couple of days before you cut and install it. OK, so that was my first mistake.
2) I didn't seal the cut ends of the BB after i cut them, again, I'm thinking this stuff is stable...ha! Seal all cut edges of MDF with Titebond III...or any waterproof glue. It dries very fast...I mean 30 seconds or less because it gets soaked in the the cut end. Also, glue the inside mitres and glue and pin nail outside mitres. Coping was not an option with this BB design and it wouldn't have helped me anyway.
3) I'd never heard of "back priming" and since the BB came primed on one side, didn't even think about it. Back prime/prime every edge. (Except the cut edges you seal with Titebond III) Yes, prime the side that faces the wall. Using a non water based primer on MDF is best (doesn't raise the grain) and I would think it'll be more moisture resistant in use.
I routed my own casings and made corner blocks from MDF for all the doors and windows (saved big $ and they look great!)...guess what...what started as a tight block to a casing...is now maybe 1/16 gap space there. All caused by what I'm certain is moisture affecting the MDF.
If you do what i suggest above, I hope it'll save you the troubles I now find myself in.
Fix for this??? I'm seriously looking at redoing all the long BB runs (i'll leave the closets and caulk the joints there) and popping off and backpriming, glueing cuts edges of all window and door casings. We're selling this house (already moved actually due to wife's medical condition), but I don't want to screw over the next buyer and want to do it right. Why not just caulk the space at the window casings and doors? I popped off the clorner blocks and reset them "tight" when the spaces appeared. I'm guessing in summer, if I don't control this now, the casings will absorb moisture and POP those corner blocks.
I made my own solid wood casings and corner blocks for the bathrooms to match the MDF ones in the house. The bathrooms ones I did prime all sides etc...due to it being in a "wet" area. Everything has stayed put.
One more tip for the corner blocks, Titebond's Moulding glue works great... Don't go crazy with it, but put a couple dabs behind those corner blocks and you'll never have to nail them. All the above advice applies to MDF corner blocks as well. Acclimatize, prime all sides etc...
Hope this helps someone out there.
Mitch in Saskatoon...back to the house now, cause time is money on this one, I'm so angry I could throw up. This is/was a BIG reno for me, wall taken out, Kitchen island biuld, solid wood drawer fronts, Stile and rail cabinet doors to match existing, Melamine cabinet construction, 100LBS drawer slides install, pantry drawers, install Cabinet crown moulding, move stove hood exhaust in attic, paint whole house, BB and corner blocks/casings etc..