The substrate is plywood.

I specifically wanted the mastic, instead of the thinset, because it's far more forgiving on a wood floor that's not stiff enough. The thinset provides no capability of movement............the mastic effectively provides unlimited capability. The floor is a single sheet of 3/4" plywood with a sheet of 1/4" luan on top..........not very stiff.

If the damn stuff would only dry.............