Varathane Diamond Spar Urethane

Has anyone had experiencing using this, or other water-based polyurethane finishes? This claims to  be designed for outside use, with UV protection, etc.  I'd be using it on the inside of my Annapolis Wherry; I'm painting the outside.

What looks attractive about this product:

  • Harder finish seems like a fine idea since it's on top of coat of epoxy.
  • Non-yellowing--an aesthetic question, but I like the current color of the wood and am not sure I need the classic yellow varnish look.
  • Water-based cleanup and presumably less-noxious vapors.
  • No need to sand between every coat... I must admit I'm getting tired of sanding. :-)

Any experience or advice most appreciated!


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RE: Varathane Diamond Spar Urethane

I'm not a pro finisher but I've had good results both spraying and brushing "water-based" clear finishes.

They're like water-based paints in that you don't have much time to brush them out. You have to get them laid on quickly and evenly and move on down the line. It's very important to plan your brushing strategy and to brush back to a wet edge.

They are more "water-white" than oil-based varnishes are are said not to yellow. In my experience, they're a little cloudy, compared to oil-base.

Unless they've upped their game recently, water-based "varnishes" (they're not truly a varnish, but an acrylic or acrylic/polyurethane blend) are less resistant to solvents, acids and alkalis than oil-based varnishes, but of more concern to a boatbuilder, they are weaker barrier to water and water vapor. They are very hard and scratch resistant.


The glycol ethers that are the solvents for water-based finishes are very toxic, but small amounts are used so there's not a lot evaporating into the atmosphere. Of the two types of glycol ethers, ethylenes and propylenes, propylenes are the less toxic.


I'm surprised to hear that you don't need to sand between coats. 



RE: Varathane Diamond Spar Urethane

Re. not needing to sand between coats--I was surprised too. This is according to the application instructions on the can, not any personal experience.

I have also been intrigued that Interlux Schooner Gold varnish application instructions say "Sand after every second coat" as long as you reapply within one month.



RE: Varathane Diamond Spar Urethane

I used the System 3 WR-LPU which is a 2-part water-based finish on my Shearwater 14.  I like it (I used satin instead of the normal gloss just because I like a satin finish)  I appreciated the water clean-up, and the less toxic qualities.  I put on 4 coats within the recommended re-coat window and did not sand except a quick wet sand with 400 grit before the final coat on the deck.  It was a bit tricky to apply if you're not used to clear finish application techniques, but if you keep the temperature low and humidiy high it's not bad.  I used a thin nap trim roller and a high quality (Purdy) synthetic brush and the final product as instructed by the excellent System 3 tech support and it looks very nice. I chose it for the less-toxic qualities as well as the reports that it holds up longer than more traditional varnish finishes.  Figured if I was going to all the work of applying it, why not use something that I have to re-apply less frequently? 


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