I am sure youre gonna get all kinds of replies to your query, my 2cents is:
To me, spar varnish is Urethane (poly) cut with something, for example, when I do exterior wood that has to hold up to spanish sun and fall rains, I use a mix of marine varnish (glossy) linseed oil and mineral spirits in equal parts, and I am not all perfect about the mix, I just eyeball it, usually more MS in the first coat. And I call that spar varnish.
I have no idea what an "exterior urethane" is but I'd guess it has UV inhibitors added. Varnish is a compound formed by cooking oil and a resin under high temps and pressure. Spar varnish is actually a "long oil" formula that goes into the cooker to make the varnish (what folks usually refer to as "polyurethane" or "urethane" or "poly" is actually varnish). Long oil simply means there is more oil in the mix, creating a softer finish that moves a little with the wood. The problem with all of this is that urethane resins don't do well in high UV environments, that's why the true marine spar varnishes use other resins in the formula. They also use considerable amounts of UV absorbents. I read a review by Flexner where he tested some box store spar varnishes…..The Helmsman did so poorly in the sun he speculated it didn't have any UV absorbents, even though the can is labeled that it does.
It's a common practice (prove it to yourself) and worthwhile, IMHO. Combine the misconceptions fed to us by the manufacturers and a wide range of results we collectively achieve with all the various finishes it's easy to be misled. I do suggest you get a book about finishes, the best (again, IMHO) is the one by Bob Flexner. Easy to read and answers almost any question about finishing you would have….plus his advice is proven. Another one would be the one by Jeff Jewitt, very much the same info but arranged in a different manner.
Well, satin urethanes have more flattening paste to take the luster out, so you end up with less sheen (as you said). Glossy urenthanes have less of this flattening paste and thus a far shinier result. Therefore, whatever is under a glossy finish is going to be pronounced; and whatever is under a satin finish will be somewhat obscured, dependent upon the level of sheen. Such is why, when using a gloss finish, it is much more critical to prep the surface well, whether its metal, wood, sheetrock, or plaster, or whatever, because the gloss finish will accentuate what lies under it! To the other end of the spectrum, there is a matte finish, which hides deformities the best of all because of the lack of sheen. This is all common knowledge to any painter worth his weight, so I cant fathom why anyone would take exception???
Apologies if there was a misunderstanding, my post was a compound statement: like he said + what I said.
It wasnt paraphrasing or parroting.
When in shop class years ago, the teacher said the gloss is the most durable finish, and that satin has basically a product like dust to make the finish lose the gloss, which makes it lose some of it's strength. So I just use gloss, and it seems to lose some of it's sheen over time.
It's all good. Discussions pop up about whether it's better to do gloss undercoats and finish with a satin if you want that sheen. The argument is you get better clarity. I suppose, but honestly I've done it both ways and don't see a difference.
I will say that for a tabletop, when I want a perfect finish, I do ten or twelve coats of gloss lacquer and then rub it back to the sheen I'm looking for. I like wet sanding with Mirka Abralon pads on my 6" dual action sander/polisher.
When in shop class years ago, the teacher said the gloss is the most durable finish, and that satin has basically a product like dust to make the finish lose the gloss, which makes it lose some of it s strength. So I just use gloss, and it seems to lose some of it s sheen over time.
I'm afraid the teacher was mistaken. The flatting material is typically silica and has no effect on the durability. In fact, gloss finishes will show scratches and scuff marks far more than a lower sheen.
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