Tips for bullnose baseboard that nobody talks about

I did my whole downstairs with these 3-piece corners last summer using 5-1/4 inch MDF base. You are spot on about the CA glue. I used the 2P-10 brand with the activator spray, and it saved me so much frustration. If you try to nail that tiny middle shim—mine were ussually about 5/8 of an inch—it just splits or disappears into the drywall mud behind the bead. One thing I ran into was that the bullnose beads aren't always a perfect 90-degree radius. I had a few spots where a 22.5-degree cut still left a gap at the back becasue the corner was blown out during the taping phase. I started keeping a scrap piece of the plastic bullnose bead in my pouch to test the fit before committing to the glue. It is a slow process, but way better than trying to glob color-matched caulk into a huge V-shape that eventually cracks anyway.

While the 22.5-degree miter is the standard geometric solution for these three-piece assemblies, I have consistently found that the width of that transitional segment must be meticulously calculated based on the specific radius of the bullnose bead. If you fail to account for the thickness of your baseboard stock, the profile geometry won't align at the apex, and even a sixteenth of an inch of excess width creates a 'ghost' gap where the board stands proud of the drywall, wich then necessitates back-planing that can easily compromise the structural integrity of your adhesive bond.

Ugh, bullnose corners are the worst. I've given up on the three-piece method lately 'cause I'm too lazy for all that gluing. I just started buying those pre-made radius blocks from the big box stores. They aren't cheap, but they save my sanity when the drywallers didn't get the corners square. You just butt your straight runs right up to 'em and you're done. Anyone else find those worth the extra cash or am I just being a hack?

I’ve run into that same issue with the mud build-up. I always take a chisel or a utility knife and scrape back the excess drywall compound right at the base of the bullnose before I even measure. If you don't clear that hump out, the middle segment of your three-piece corner will tilt outward at the bottom. It ruins the vertical alignment and makes the top gap look huge. Getting it back to the bare metal bead makes the assembly sit way more flush against the wall.