Cat urine smell in plywood subfloor

Q: We are putting down a Pergo laminate floor in a den which had carpeting. The carpet and pad was wet with cat urine for a number of months and it soaked into the plywood. The wood has dark spots, but appears to be fairly dry.

We used baking soda and an odor digester to remove most of the smell at this point. You can only smell it if you get very close to the wood. This is a first floor room over a basement, where the washer and dryer are located.

My question is: should we install a vapor barrier, paint the floor, or take some other action before installing the floor?

A: I would coat it with shellac. Zinsser makes a couple of products like this which are excellent ‘blockers’. They dry fast too.