Q.
When finishing my woodworking projects, I have always made it a point to seal all accessible surfaces (whether they were exposed or not). Cabinet backs and drawer bottoms are sealed front and back, top and bottom.
I was taught that properly dried wood, after manufactured into a finished piece, should be sealed with a protective coating to prevent the wood's moisture level from changing with the surrounding environment, and to protect it from contaminants (dirt, grease, stains, etc.).
I have heard other finishers claim that wood finishes (lacquers, varnishes, urethanes, etc.) are porous and allow the wood to breathe, so sealing the entire project is not necessary. Backs of cabinet backs, bottoms of drawer bottoms, backs of face frames, etc. can be left exposed.
I believe this to be incorrect and will/could cause possible problems from moisture level changes.
Does anybody have any feedback on this subject? References? Sources for further information?
And would the finishing requirements change if the project(s) were made with different materials, i.e., solids vs. veneers laminated on plywood, MDF and/or particleboard substrates?
You cannot stop moisture absorption in wood but you can slow it down with a properly applied finish. An example of this would be spraying two coats of finish on one side of a board and no finish on the other side. The side with no finish will absorb moisture faster than the finished side, causing a bow or cup.
Wood constantly moves, regardless if there is a finish on the piece or not. The key is to plan for movement in the design.
You are correct in saying moisture level changes will cause problems in wood. If you have a film-building finish on one side of a piece and no finish on the other, the side with no finish will absorb the moisture faster. Problems may therefore arise in glue joints, with the continuous expansion and contraction of the wood.
In my opinion and experience, you're always better off putting a finish on all surfaces if you can. The exception would probably be the back of a cabinet or underside of a drawer made with veneer plywood.
I live in Florida and constantly deal with relative humidity.
While finish won't stop all moisture penetration, it WILL equalize your work on both sides, eliminating or reducing warpage.
