Why is my baseboard gapping when I try to caulk it?

I run a lot of MDF base and the corners are always where the trouble shows up, so I've been down this road plenty. What you're seeing is ussually not the trim moving. It's the wall. Drywall inside corners are rarely actualy square, and even if the base sits tight against the drywall, there's a hollow spot or a bow behind it that opens up once you look at it from an angle. Door casings look fine because you're caulking against jamb material that's dead flat and rigid, not a wall that waves in and out by a sixteenth here and there. MDF does move a little with humidity, but on baseboard it's mostly the length that changes, not the profile, so it's rarely what's opening a mitre or a cope. If the gaps are along the top edge where the board meets the wall, that's almost always the wall not being flat. I scribe or shim for the worst spots before anything gets nailed off permanently. For the caulk itself, the trick is a small bead and a wet finger. Cut the tip tiny, run a thin bead into the gap, then drag it smooth with a finger dipped in water so the excess wipes off onto your rag instead of smearing across the paint. If you try to fill a gap that's too wide with caulk alone it'll shrink back and look sketchy no matter what you do. Anything over about a sixteenth wide I'd rather fill first and caulk over. One more thing on pre-coped stock: those copes are usually cut to a theoretical perfect fit, and no inside corner in a real house is perfectly square. Even a good cope needs a touch of filing or a shaving taken off the back angle to sit tight. I check every cope against its actual corner before it goes up, because the factory one rarely drops right in without a little help.

If the gaps are small and consistent, a thin tooled bead will look crisp once it dries. If they're wandering in and out, it's the wall, and no amount of caulk will hide it from a raking light.

I don't buy the idea that every factory cope needs some serious shaving taken off the back angle to make it sit. It feels like that assumes the worst possible stock, which isn't always the case. I've had plenty of decent runs of pre-cut material where the miters actually come out close enough that you just need a little gentle tap and it settles in tight.

Instead of trying to cut the profile down, if you get those tiny fit issues, I usually just take a fine rasp or some very light sandpaper to the *edge* where it butts against the jamb. That way, you take just enough material to close the gap without ruining the intended cope shape. It keeps the trim looking right and doesn't turn installation into a major sanding project.

One thing that bit me on a 60-foot run last spring was cut ends soaking up moisture. Every cope and butt joint I made on site was raw MDF, and a couple swelled just enough to push the joints open after a humid week before paint. Now I hit every cut edge with primer before assembly, takes ten minutes and the joints stay put. Also try checking your corners with a raking light before you caulk anything, hold a flashlight low along the base and every gap lights up like a runway.

One thing I learned the hard way over the years is don't caulk the same day you nail it off if the house is dry and the heat's running. I'd fill everything, come back the next morning and the bead had cracked in a couple spots. Now I let the trim sit a day or two, then caulk before the finish coat goes on so the paint does half the hiding for you. Also, back in the day we'd run a putty knife along the wall at the base line to find the dips, then float a little joint compound on the floor edge before the base ever landed. Old trick but it still beats fighting a gap with a fat bead of caulk.

Quick update — I decided to let the trim sit for a day or two before doing the final caulking. Following one of the replies about not sealing it imediately, the joints had time to stabilize. The caulk lines are much crisper and the gaps I was seeing are totally gone now.