At our Fine Woodworkers Club meeting, one of the members said that polyurethane will eventually delaminate between coats. Is this true? I’ve got two pieces whose polyurethane finish is over forty years old and they are in fine shape. One of them has been in a Texas attic for ten years, experiencing wide temperature and humidity variations.
I don’t want to contradict him or start a dispute. I just want to know what longevity I can expect from polyurethane.
Replies
Never heard that. As long as the surfaces were adequately scuff sanded between coats, poly varnish should not fail that way. But polyurethane will "delaminate" from the wood if the finish has been exposed to direct sunlight and moisture.
I don't want to contradict him or start a dispute. I just want to know what longevity I can expect from polyurethane.
GO AHEAD and do it.. Just make sure you payed for the beer he has while 'chattin'
EDIT:: I forgot.. Tell him Varnish sort of do that if used in salt water and maybe he was mixed up... LOL...
Edited 8/6/2005 3:26 pm ET by Will George
It depends on how much time there is between coats. If you're just letting each coat flash off before applying another coat (so that you don't have runs or sags) then you don't need to scuff between coats. If each coat is allowed to cure out then all of my experience as well as everything I've read says that Howie is right and all you need is to scuff between coats.
I will note here that all of the Product Data Sheets that I've read say that only slight abrasion between coats is all that's needed. The automotive ones (catalyzed polyurethane) say that you can actually ruin intercoat adhesion by using too course of a grit to scuff between coats. I'm at home and don't have any of them handy... but, my recollection is that 320 - 400 is about as course a grit as you want to use to scuff between coats. DuPont recommends up to 600 grit for scuffing between coats.
As for longevity... there are many factors involved. But, I would think that the longevity of a clear poly finish on a piece of furniture which is kept inside would be measured in decades. While Poly typically doesn't exactly like to be exposed to UV... it will withstand UV better than almost anything out there. So, I would think that polyurethane would probably give you the longest longevity out of all your options.
BTW... anyone know how Shellac is rated for UV rays? I don't believe I've ever read anything which says one way or the other. I tend to think of Shellac as having many of the same properties of lacquer. But, perhaps that's not a safe assumption to make with regard to UV rays since lacquer degrades very rapidly when exposed to UV.
"Rather fail with honor than succeed by fraud" - Sophocles.
Kevin,
The first coat is a grain sealer and I sand it and the grain nibs with 240 grit. The second coat is sanded with 320 grit. The third coat is sanded with 400 grit. I sand pretty aggressively to avoid getting a "plastic" look. On highly-figured wood, it sometimes takes six coats to fill the areas that drink the finish and look dry. Does this sound okay to you?
You can also avoid the "plastic" look by using a Satin finish.
You should also stir poly thoroughly before using. Some of the "goop" in the bottom of the can is there to dull down the shine in the finish. You can turn a satin into a semi gloss if it isn't stirred thoroughly - lol.
As to problems with poly, I've never experienced any. I made this desk six years ago and it sits by the window getting full morning sun. It's pretty dusty, and there are a few coffee cup rings on it, but the finish will look like new when (if?) I get one of those round tuits and clean up this dump - lol.
Edited 8/6/2005 10:02 pm ET by Dave
A satin sheen is an excellent way to make a finish look more natural. But, it's a bit of a two-edged sword in some cases. With figured wood the conventional wisdom, Jeff Jewitt talks about this I believe, the flatting agent in a satin sheen finish will partially obscure the figure, which would be somewhat counterproductive.
I learned this the hard way on an antique vanity I refinished several years ago. It was a shabby old "waterfall" style vanity which my father-in-law gave me. I ended up using some curly (although not heavily so) Limba veneer as an accent in a couple areas to basically cover up problems. Then I finished it with a satin pre-catalyzed lacquer. The Limba looks great. It's some verigated black and white Limba veneer which has a nice inherent contrast to it. But, the figure is almost completely unnoticable. This was a couple years before I read Jeff Jewitt talk about how the flatting agent can obscure figure in wood. So, it seems to confirm what he said.
That said... the Limba I used wasn't heavily figured. And I've never tried that approach with any other figured wood. With something like a quilted Maple or something of that order of figure I don't imagine that even flatting agent could begin to obscure the figure. And I know from personal experience that tightly curled Maple and other similiar woods still look curly even under a flattened down finish.
What Jeff Jewitt recommends on figured woods is to finish with a full gloss and then hand rub it down to whatever sheen is desired. The full gloss finish material doesn't have flatting agent and thus, according to him, the end result won't include opaque flatting materials sitting between the figured wood and your eyes. It makes perfect sense. Although I've never tried it. On my own personal projects I have chosen to go with full gloss lacquer over figured wood. On stuff that I do for my employer I use satin because that's what the clients almost always want and if it obscures some of the figure... well, they don't pay me to second guess clients. LOL
Preemptive Karma
"Rather fail with honor than succeed by fraud" - Sophocles.
Well, my first thought is that if it works for you and you've established a track record of doing it a certain way and haven't had any problems... then it's really not up to me or anyone else to say whether it's okay or not. Which is to say that it's pretty hard to argue with an established track record.
That said... I do think there is an inherent difference between using a polyurethane on wood as a finish and using a polyurethane over basecoat paint on a car. The plastic look is precisely what automotive refinishers are looking for. Although we don't tend to think of it as looking like plastic per se. But, in an automotive application, or any similiar type of application, that super smooth, flawless look is the goal. Whereas I can see why some may not like it over wood. That's really a highly subjective thing. Some folks do what a super smooth, almost plastic looking finish.
Just a couple months ago I did some samples for a client using curly Maple and curly Sycamore (spelling?) veneers. They wanted two sets. One set they wanted the grain completely filled. I used a super high build Italian-made polyester product to fill the grain completely, sanded almost back down to the veneer so that it was flawlessly smooth, and then topped that off with a Vinyl conversion varnish. The other set just got a sealer coat, scuffed moderately, and then topcoated... all with the conversion varnish so that the grain wasn't even close to being filled.
Have you thought about rubbing out the poly to a satin or lower sheen as a means of avoiding the plastic look?
Preemptive Karma
"Rather fail with honor than succeed by fraud" - Sophocles.
Hello Kevin,
Thanks for your reply. I avoid the plastic look by having the last coat show some of the wood texture and using satin finish. I like the feel of the wood when it is somewhere between plastic and velvet, just about the smoothness of a woman's skin. (Hope that's not going to get me into trouble here!) I've made walnut boxes this way for my wife and adult daughters. Their first impulse is to stroke the finish and they like the feel of it.
My main concer was wheter this finish method was going to last. It appears that it will last a LONG time.
John... you may not have easy access to it because it's strictly a commercial coating. But, the one finish that I've worked with which consistently gets rave reviews for how silky it feels is Sherwin Williams Vinyl Conversion Varnish. I dunno what it is other than it must be the Vinyl resins they use. But I've never ran my hands across a finish which yielded such a wonderful tactile feedback as this Vinyl finish.
Preemptive Karma
"Rather fail with honor than succeed by fraud" - Sophocles.
Hello Kevin,
I visited the Sherwin Williams website and the only "conversion varnish" was "water white conversion varnish"
http://www.sherlink.com/sher-link/application;JSESSIONID_WLCS_COMMERCE=C2bnfyWlMjG6jmrJTP2ZwWsyjRp10cdL2R1qwhLSvzpppQYTQFDN!591829993!972024821?namespace=productCatalog&origin=minisearch.jsp&event=link.details&wlcs_catalog_item_sku=5204359&onlyDetails=true
Since I'm just a "brush boy" I'll follow your advice and stick with what works for me.
Best regards,
John
Edited 8/7/2005 3:12 pm ET by John Huber
Unfortunately they don't list the one I was talking about. Perhaps it's being phased out. They do list a close cousin which is also a vinyl resin conversion varnish: http://www2.sherwin-williams.com/chemicalCoatings/datapages/CC-F8.pdf The other one is designated as "M" vinyl conversion varnish and it's designed to build like a normal finish. The linked "C" vinyl conversion varnish is a specialty finish which I have used. It is designed to produce a finish which isn't readily apparent. It'll look like there is no finish at all on the wood when in fact there is, but it's soaked into the wood. If you like the look and feel of an oil finish but want the durability of something a bit tougher, the SW "C" is what you'd want to use.
Preemptive Karma
"Rather fail with honor than succeed by fraud" - Sophocles.
Hello Kevin,
When I graduate to spraying, that looks like a great finish to use. Thanks for the tip.
Best regards,
John
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