The tail of piano was cracked due to environment and released tension after strings were taken off. The shop owner suggested using of WoodEpox to fill the crack. Before filling, the bottom board, where the tail leg is attached, is taken off. The filling was done on day one, then sanded on day two when it is completely dried. Note: leave the filling higher than the surface because it will shrink when it dries. Turn vacuum on while sanding WoodEpox. It is very powdery, possibly bad for the lung, possibly very bad.
We used 80 grid sand paper to smooth out surface. The flat paint does not require the surface very fine, such as soundboard. Once the piano is turned right side up in home/at school, the bottom will not be seen anymore. Check that all painting area is sanded.
Before painting, tighten all screws.
With a 3" foam brush, carefully paint the piano bottom surface and parts taken off from the bottom. The paint dries in a few hours. Reattach all parts before flipping piano right side up.