: ...polyurethane IS varnish. The difference is that it uses polyurethane resin
: rather than alkyd or phenol resins. It is the same type of finish in that
: it cures through a chemical with oxygen in the air and the cured finish
: cannot be disolved by the thinner that is used in the uncured product.
: Multiple coats do not chemically bond with each other. Cured polyurethane
: is very durable. There are other differences in it's properties vs. other
: varnishes. For more details, pick up a copy of Understanding Wood
: Finishing by Bob Flexner; it's a very interesting read.

: FWIW, prior to reading the book, I thought that polyurethane was some form of
: plastic finish and that varnishes were something different. I also thought
: that oil finishes were waterproof and that mutliple coats of oil would
: re-dissolve each other and blend. The book dispells these common
: misonceptions and explains how finishes work and what the pro's and cons
: of each are. It explained some problems I had run into and has prevented
: me from making mistakes in the future. For me, it was definitely money
: well spent.

: Speaking of price, Woodworker's Warehouse has great prices on books.

Hi Brian,

Thanks for your explaination. I will paste an explaination I received from the Minwax Board below and then I have another question.

""Re: Urethane vs. Varnish

By definition, spar urethane is a type of varnish. But not all varnishes are urethane based.

For instances, Minwax Helmsman, a spar urethane, is indeed a urethane varnish with UV absorbers and flexible resins that allow the finish to expand and contract with temperature changes. The same is true for Minwax Clear Shield, which has even more UV absorbers than Helmsman does.

It's important to remember that for boats, kayaks, etc., that are immersed in water, you need to use a marine varnish. You also need to be prepared to recoat with the marine varnish annually, just like you would with any other exterior wood that is exposed to water, temperature changes, etc.

Hope this helps.""

I have asked this question on the Minwax Board and have not yet received an answer, "Can Minwax Helmsman Spar Urethane be used on my kayak or do I need to get a varnish that says MARINE VARNISH? Also, a quart of Helmsman cost about $7 while some of the varnishes used on this board are about $30. What is the difference?"

John