On Saturday, August 1, 2015 at 10:33:35 AM UTC-4, Leon wrote:
On 8/1/2015 9:07 AM, John McCoy wrote:
Leon lcb11211@swbelldotnet wrote in
:

FWIW "varnish" is pretty much a generic term to describe any clear
protective finish including shellac, lacquer, etc. Basically a resin
dissolved in a liquid.

Well, the Oxford English Dictionary agrees with you (and traces
the word back to a Greek word referring to a place in Asia Minor
where tree resin was collected).

But like Mike, I think in modern usage shellac and lacquer are
considered different, and varnish refers to the finishes which
have a polymerization reaction - polyurethane, epoxy, etc.

John


I'm not real sure if you could find a printed modern definition that
states your last paragraph, or something like that.
But I will agree that varnish is seen as something more specific.
Today a varnish is often simply called by its make up.


This site seems to do a decent job of differentiating between varnish and polyurethane. Well, at least is seems like a decent job to a rookie like me.

Stolen without permission from:

http://www.shesails.net/tag/varnish-vs-polyurethane/

Varnish:

Varnish is made from a combination of resin (either plant-derived or synthetic), drying oils (most often tung or linseed), solvents (turpentine, etc.), UV protectors, and sometimes driers. Traditional knowledge says that varnishes give superior UV protection to wood that gets sun exposure, and a relatively long-lasting, flexible finish. Varnish's flexibility makes it ideal for moving parts which might cause other harder finishes to crack, allowing moisture to damage the wood.

A proper varnish job requires a somewhat experienced hand, not only for a nice-looking finish, but also to ensure that the finish takes. If done improperly, varnish can crack, peel, delaminate, bubble, or never fully harden. Varnishing also requires a somewhat dust-free environment, nice weather, and lots of time to apply many coats. For our purposes on Clearwater, we tend only to use varnish on exterior wood that has a low likelihood of getting banged or chafed, such as hatch covers, since touch-ups are somewhat challenging and time consuming. Someone who knows what they are doing should scuff and apply a top coat of varnish once per season.

Polyurethane:

Polyurethane is a finish/sealant that has a very similar application and appearance to varnish, and therefore the two are often used interchangeably. Polyurethane differs from varnish in many ways that don't matter to us very much, namely the way it is made, and the chemical process it goes through to "kick". What IS relevant to us is that a polyurethane finish tends to be MUCH harder than varnish, meaning it holds up much better under certain circumstances. The down side is that it also tends to be less flexible, which means that it will crack much more easily on pieces that flex.

A point about UV protection: I've always been told that the difference between varnish and poly is that varnish protects against UV damage and poly doesn't. From what I gather, this isn't fully true, at least anymore. Both varnish and poly CAN contain UV protective additives, but from what I've read, over the long haul, the UV filters in polyurethane break down faster than those in varnish. I have not myself worked on the same piece of half-varnished and half-polyurethaned wood for 10 years, so I can't speak from personal experience.