Both the recycled materials I used – MDF and carpet underlay - can be prone to decomposing. To avoid decomposition limiting the efficiency of my speakers both materials are exposed to only minimal air. The three layers of both primer and top coat I sprayed onto my cabinet effectively seal the MDF and ply from the elements. This greatly increases the lifespan of my speaker cabinets.
The carpet underlay is also sealed in a mostly airless environment. The cabinet is airtight aside from the ports, which cannot be avoided. It is unlikely that the small amount of air drawn in though the ports when the speakers are playing will cause a lot of decomposition. So the carpet underlay is relatively protected too.