What to use to waterproof interior of stand?

Check out RedGard. http://www.custombuildingproducts.c...paration/membranes-underlayments/redgard.aspx

It goes on pink and turns very red when it is cured. I don't know if you can paint over it--you'll have to ask an expert. It is used to seal under showers, floors and in roofing applications. If you can live with the red color, it might work for you. It is very red.

We sealed our cement/wood flooring with RedGard before we tiled. It was a ghastly red.

I just used a white deck and fence stain. Supposed to last for 25 years outdoors, should be good enough.