Background Reading: 6-O: Frame Rot
You’ve already checked the exterior frame and base (7-G Cabinet And Bottom), but with the panels off, you can now check the back of the panels and the frame of the hot tub.
-If the exterior skirting is synthetic, check to see how the slats are held together at the back (are they stapled/glued to pressboard or plywood that’s deteriorating?
-Check out the vertical framing, especially the bottom ends) for wood rot. Often, they’ve deteriorated completely because they’re usually not pressure treated.
-if possible, check out the bottom wooden rails that run along the very bottom of the hot tub, in the middle. Those are the ones most likely to rot, because in our wet climate, water wicks inside and never dries out all year long.
-use a screwdriver or keys to poke at the bottom rail, to test for punky/soft wood, indicating rot.