Planning on rebuilding hot tub siding and have a question about it.

[d.sebens]

My siding appears to be pvc(or something not wood). It is held together with strips of osb on each section. Some of it has gotten rotten and some the glue has detached from the siding. I plan on repairing the frame and repainting the siding. 
 

My first question is if I should do framing strips in a box shape as they did before or if I could do a solid plywood panel behind the siding?  My thought is if it’s solid there is much more glueing surface and should hold better. But I’m also thinking that I could possibly have issues with expansion and contraction. Which if we’re being honest I don’t know if the box frame of strips does anything for that?  I don’t have a picture of the back of the panel but I do have one of the outside and I’ll sketch up what the inside looks like. 
 

My second question is can I just scrub it and paint over the siding?

The sketch uploaded is an idea of how it is currently framed.

Edited to add what the framing looks like.

Edited May 6 by d.sebens

[Cusser]

My spa is so old (1988) it has real wood panels.  But they were not made well.  I ended up replacing the vertical boards with new redwood.  Picture here is with original vertical slats, I replaced with like 4" boards and reinforced on the inside surface.