The walls are insulated with fiberglass insulation covered with 35 year old exterior grade 1/2 inch OSB that's glued and screwed to the studs. It's got a wax type layer that would need to be sanded off to paint. Contractor quoted 3.5k just for spraying the walls...no prep...no cielings. The wall would never be anywhere near smooth like it was. 1/4 inch sheet rock is cheap enough at 14.60 per 4 x 8. Only needed 23 of them. The center horizontal line, running around the garage, will be covered with trim after finishing. Corner trim around the ceiling / wall and in the corners.
Wavy would be better than chips of peeling OSB and the plywood texture. Garage is insulated and dry...with heat and, soon, AC.