Common problem-drywall butts against trim, in some cases being taped and mudded to it.
Part one of my solution: foam in place caulk.
Part two of my solution: trim foam and smooth out with painters caulk.
Beginning to paint.
Renovation shortcuts are normal in houses this old. The last time the living room was done-under deadline pressure-drywall was butted up against the trim. This is because the walls had been lath and plaster, and the drywall did not match the plaster thickness everywhere. It wasn't possible to strip the trim and rebuild it either. This left gaps which were filled with a combination of caulk and drywall mud-sometimes with the tape and mud lapping up onto the edge of the trim. This worked fine behind wallpaper, but is a problem for paint.
Once you scrape out the caulk and joint compound, you are left with a crack, and usually a large air space behind it. The air space means that whatever you do will be unsupported. My solution is to fill the gaps with foam-in-place insulation, then caulk over it after trimming. The foam is more than hard enough when cured, and its adhesive qualities help glue down the edges of drywall and any wiggly bits of trim from the inside.
Dale Austin 2006
All images and text Copyright Dale Austin, 1962-2008