use a permanent varnish to seal the paints (varnish protects the surface). A word of precaution though, make sure that you are satisfied with your painting cause as soon as you applied the varnish on top you CANNOT redo your painting anymore. Also, Acrylic and oil has a DIFFERENT varnish -You CANNOT use the same varnish for both acrylic and oil paintings.

Responding to the other answer, i WOULD NOT put the varnish on the back to prevent the wood from breathing because you might also trap the fungus spores that is inside the paint causing the mold and the fungus to grow from the inside out (and that would permanently damage your painting, since you cannot wipe it anymore after sealing the wood with varnish)

instead try this additional method:
store your paintings in a clean and dry environment (and garage is not the best place to put them). Fungus only grows in a moist environment. Either you put the painting in a damp place, or the wood or the paint itself is wet. If the wood that you are using is pretty thick, you can try dry it using sunlight with the bottom side up (Do NOT put the painted side directly to sunlight).
A word of precaution, make sure you watch the painting while it dries (you want to monitor it).
Because, if the wood is not thick enough, drying it using sunlight will cause your paingting surface to buckle. That’s why you have to check on your paintings every once in a while, so that you can stop the drying process if the TINIEST sign of buckling start to present. Hope that helps.