Tips on Sealing Wooden Art Panels and Resealing Exposed Edges
For do-it-yourselfers wanting to seal the surfaces of wooden panels and provide a meaningful, effective, and durable vapor barrier, we recommend the use of urethanes.
A product that can be bought in any home center, hardware, or house paint store, is pure spirit-based Polyurethane Varnish. Don't buy the Glossy finish; it may not provide an adequate texture for good paint adhesion of your primer. Buy Flat finish whenever possible, although it is usually quite hard to find. Settle for Satin finish as an alternative. Do not buy "water-based", "water-born", or "water-clean-up" urethane or polyurethane; it does not provide a tight vapor barrier. Also, do not buy "marine" or "spar" urethane or polyurethane; these are generally alkyd-modified urethanes, which can cause durability problems.
Pure spirit-based Polyurethane Varnishes should provide a good, tight vapor barrier with the application of two or three coats. They cannot provide as strong or tight a vapor barrier as industrial type urethanes, but those coatings require a factory environment and equipment for application.
The trick to applying this type of coating is to be sure to provide an adequately "toothy" surface for your primer to grip. A surface without sufficient tooth or texture will sow the seeds for future problems for your painting. We believe that the best way to do this is to sand the wood so that enough texture is left that it will not be completely filled in and smoothed out by the varnish sealer. If the surface is still sufficiently rough after the sealer has been applied, then your primer can get an adequate grip. Of course, this means applying and sanding more coats of your primer if you want a very smooth painting surface. (Be sure not to sand through the sealant coat when sanding your primer!) An easier but less satisfactory method would be to apply the sealer to a very smooth surface, and then sand the sealer between coats (in this case, a minimum of 3) and after the final coat, being sure not to sand deeply or roughly enough to break through the coating.
Even the best sealed surfaces will not protect your panel from deterioration if the edges of the panel are not thoroughly sealed as well. The edges of a panel are invariably highly absorbent and very difficult to seal properly. Trying to seal them can feel like trying to seal a sponge. Even many, many coats of polyurethane varnish will not seal the straw-like fibers exposed at the panel edges.
Using caulking sealants helps to seal the edges, since they will plug the fibers, rather than being sucked into them. Conscientious sign painters caulk the edges of their panels to increase their durability. Caulking sealants, because of their thickness, can meaningfully slow down the exchange of moisture vapor in and out of the wood. Silicone is a very good sealant; acrylic and vinyl caulks somewhat less so. However, GE, one of the largest makers of caulking sealants, says that even silicone sealants are not vapor-proof barriers. There are polyurethane caulks which may work even better in this regard. Note however, that they are very sticky, and quite tricky to apply.
A more practical approach is to use a "paintable" acrylic or silicone-fortified acrylic caulking. Apply this before you seal the surface of the panel. Then apply the polyurethane varnish over the caulked edges. The caulking will plug the wood fibers, and the polyurethane varnish will then provide the neccesary seal.