Replies

1. 
   Senomozi | Oct 28, 2007 06:03pm | #1

   Hello John,

   The term "varnish" is just a generic name to refer to a solution that contains a solvent and a resin.  When you apply the solution a chemical reaction takes places leaving only the resin on the surface.  Polyurethane, urethane and alkyd based finishes are therefore all varnishes.  Simply use the one best suited for your application and your preferences.  Color, sheen, oil or water based, UV resistance, interior/exterior are part of the equation.   

   Shellac is not a varnish but it can be used as a top coat.  However because of its characteristics it typically is not used on surfaces that will be subject to abuse like the top of a kitchen table.  There might be a bit of debate over the usefulness of shellac as a top coat but there is no debate over the fact dewaxed shellac is well suited as an intermediate coat.  For example, sealing knotty pine before applying a top coat of varnish.

   All that being said the best advice I could provide would be for you to get a book on finishing to demystify all this.  I personally own "Understanding Wood Finishes" by Bob Flexner.  The book does exactly what its title says.  Now I understand why you need to sand between coats of poly but not required between coats of lacquer or shellac for example.  Unlike woodworking, finishing is not intuitive unless you are a chemist, hence the importance of informing oneself.  That book really delivers from that point of view. 

   Regards,

1. 
   Libraryroom | Oct 28, 2007 06:42pm | #2

   Dear Senomozi,Thank you very much for the in-depth response. The book will be very helpful for my future projects.Regards,John

2. 
   SteveSchoene | Oct 29, 2007 12:49am | #3

   I second the idea about reading the Flexner book. 

   The description of varnishes is not quite right, though the essence is correct.  Varnish made by the manufacturer by reacting an oil and a resin, usually with heat, into a new compound called varnish.  Polyurethane is one resin, and alkyd and phenolic are others.  In DIY single part finishes if polyurethane is used it is usually in combination with alkyd.  All of those products are equally varnishes.  There is some thinner in varnish to make it easier to brush, but it's not a true solvent since after if evaporates and the varnish continues to cure by a oxidation process that links the varnious molecules together, the thinner, usually mineral spirits will no longer dissolve the varnish.  That does mean that you need to sand between coats for the new finish to have something to grab to when it cures. 

   Shellac is a resin mixed with one of its solvents, almost always alcohol.  When the alcohol evaporates the resin is left on the surface.  But alcohol will always redissolve shellac.  The other principal solvent for shellac is ammonia.  It's not much used in finish it self, but can be used to clean brushes and the like.  Ammonia will also damage dried shellac.  The property of shellac to redissolve in its solvents is one of its vulnerabilities.  But that same property makes any damage very easy to repair.  It also means that since each new coat dissolves into the old surface shellac doesn't have to be sanded between coats.