Varnish Bed Slats

I had the beginning of fungi growing between my mattress and the wooden board it lays on due to moisture getting trapped inbetween. I have treated the fungi and cut holes and lines into the board to make air circulation possible and make space for the moisture to escape.

My question is now if I should additionally varnish the wooden board to prevent moisture uptake or is it better the let it be able to "breathe" ?