Scrape

the Imagination House
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If the floor is tight in the groove, scrape with the back edge of a utility knife at a slight angle to get the clearance you need for a good fit
Paint sneaks onto the edges of the grooves and makes them narrower. It's not a drip, and often it doesn't even look like anything at all - but the jelly-thickness of good paint can leave it hanging over the edge of a groove like the crest of a snow-drift, and it can be enough to press in on a floor and not allow it to fit into the groove. At the same time, paint adds thickness to the floors (or dividers). The thickness of paint doesn't seem like much, but it adds up and can make the assembly too tight. It's easy, though, to scrape the groove and to scrape for clearance on the edges of the ceilings. I generally prime or paint the inside of a dollhouse before assembly (one coat and sand down to the wood), so this step is part of almost every build.