We have a 3 season screen room with knotty pine walls and ceiling. We've experienced mold on the wood which we have stripped off. We would like to prevent going forward. What should we do? We've been getting conflicting opinions
1. Use natural semitransparent stain only. Don't use polyurethane.
2. Use stain then 3 coats of polyurethane
3. Use marine deck varnish only
4. Water based vs oil based vs latex stains

Which will provide the best protection
How long should these last? One said 5 years, one said 15 years