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Home > Finishing

How to choose the right finish, Part 3

Now let's look at protection and durability of finishes.

Protection:

Most wood looks nice completely unfinished. So why do we even
bother to go through all the steps necessary to finish a piece of
furniture?  Two reasons are:  To protect the wood and add to it's
durability.  

The best reason to protect wood is to slow down the water transfer
between the wood and it's environment. Wood movement is caused by
moisture transfer.  When wood has a high moisture content it
swells, mostly across the grain.  The opposite is true when wood
loses moisture.  This movement can play havoc on glue joints.  

For a finish to be resistant to water and water vapor a finish must
be fairly thick.  The best finishes to use for this are varnishes
and water based (there are others but they must be sprayed on).
Shellac does a good job against water vapor but a poor job against
direct water contact. Wax and oil finishes do the poorest job
against water and water vapor mostly due to their thin coats.

No finish will completely stop the transfer of moisture between
wood and it's environment, some just do a better job of slowing it
down.

Durability:

The durability of a finish must take into consideration wear
resistance, solvent and other chemical resistance, and heat
resistance.
 
Varnish and water base finishes do a good job with wear resistance.
Shellac and brushed on lacquer do a fair job.  For finishes that
can be applied by hand, varnish is the only one that gives good
chemical and heat resistance.  

In our next article we'll look at ease of application, health
concerns and ease of repairing.

Craig Stevens
Woodworkers Resource