We frequently see the term poly used as the sole description for a finishing material a lot. But, it really makes no sense. And, in the forum, using just the term poly prompts a question–is it oil based or waterborne?
Poly(urethane) is just a kind of (plastic) resin used in finishes, it’s not really a kind of finish. What we most often see called “poly” is a single-part oil based varnish which has polyurethane resin added to alkyd resin. (The other resin used in single part oil based varnishes is phenolic. It’s tougher than alkyd, but darker in color and yellows more.)
The addition of polyurethane resins in these single part varnishes makes no fundamental change–a little more abrasion resistance, a little less clarity, a bit greater tendancy to adhesion problems. Perhaps the most significant effect of adding polyurethane to alkyd resin in varnish is that it makes the finish a little more difficult to rub out with abrasives to an even sheen. If looking for a short form term it makes more sense to just call the oil based finishes “varnish” with the qualifier of poly added if appropriate.
Polyurethane resin may also be added to acyrlic resin which predominates in waterborne finishes. It also adds a bit of abrasion resistance. But waterborne “poly” (which is essentially an acyrlic resin finish) has almost no basic functional characteristics in common with oil based “poly”. It is just confusing to also call these “poly” when the important characteristic is that they are waterborne.
Of course, there are also finishes that have 100% polyurethane resin. But these are all two-part (2K) finishes, and each part contains one of the basic components that combine to chemically create polyurethane, a polyol in one part and a isocyanate in the other. There are a great many different forms of polyurethane that give a wide range of operating properties. Some are quite “bullet proof” paints used in industrial applications. But because of the toxicity of the components, you need a safe place to use 2k polyurethane, usually a spray booth. I know of only one company that markets a 2K polyurethane to the DIY market. At least as a few years ago, there is a 58 page instruction manual, as well as a DVD, on it’s application methods. Calling this kind of finish “poly” would be more correct, but the 2K poly is so different than the single part finishes with a bit of added polyurethane that most would call them 2-part or 2K polyuethanes.
Marketing folks would have us believe that the addition of a little polyurethane resin makes a finish quite like the 2K industrial finishes. Not so.
Pardon the rant.
Replies
This is exactly the kind of information that "we" can use. Keep up the good rants!
You know, it would be a lot easier to educate oneself about finishing products if the ingredients and proportions were ever listed on the cans. But they're not and so we end up talking about brand names and us amateurs don't really know what we are using . . . That's my rant, care to add?
Brian
Add to that the consumer that seems to equate "clear finish" with "polyurethane." I get tired of hearing, "I got this table from the furniture store and the polyurethane on it..." I assure them that it is most certainly NOT polyurethane.
And for the finish-phobic craftsman who believes, what ever the question, Poly is the answer.
I've always got into arguements over my use of Polyoneverythane. The person will complain about how cloudy the finish is. I ask "How many coats?" They reply, "10" and I say "No wonder". The "trick with Poly..... is to brush on as few thin ( not thinned) coats as needed. Otherwise, too many elements get added into the "mix" and the piece looks encased in plastic. They'll complain about how "soft" Poly... is. It's "soft" to MOVE with any wood movement that will be going on over time. The ONLY time I use a "thinner" with the Poly... is to clean the brush when I'm done. Just my $.02 worth.
For interior uses, the traditional resin varnishes have plenty of flexibility to deal with the wood movement that one would expect over time. After all, all these varnishes are much more flexible than lacquer, until recently, the standard commercial furniture finish for decades. But, the softness does make the rubbing out process more difficult, especially if done after a full cure to avoid problems of shrinkage, and to assure maximum hardness.
As far as clarity goes, you are right that with thin coatings the haziness associated with polyurethane is almost indistinquishable. But, if three thin coats are optimal for uraklyd varnishes, then four thin coats might be acceptable with non-poly varnishes, and provide greater durabilty in all dimensions except abrasion.
In the last 10 years or so, as VOC regulations have come into the fore, thinner has been removed from varnishes and almost all of them will perform better, and require less finesse in applying, with the addition of small amounts of thinner. Thinner doesn't participate in the final finish and the resulting film has the same properties. But instead of say, 1.10 dry mil thickness, a 5% thinning might leave you with about 1.03 or 1.04 dry mil thickness. (You may spread it further so the wet mil thickness might be slightly less too.) But if it makes it easier to avoid air bubbles you are far ahead.
"Pardon the rant." Not at all, rant on. Great post, Steve! The marketers have a great nack for completely obscuring finishing facts. Anything that cuts through the fog is welcome.
I have wondered the same things but not even close to being as clear as you.
The thing that "flipped the bit" for me was when water based finishes started to be marketed as Poly. My head hurt thinking about it.
Another finishing question that has been spinning my head lately is which brush to use for shellac. I now know that the correct answer is a high quality natural synthetic bristle brush. To see why I conclude that put "shellac brush" into Google and read the top responses.
Frank,
natural synthetic bristle brush
Not directing this at you but that seems like more double talk - I wonder what is natural synthetic?
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
The double talk is what I was pointing out.
One expert says to use the best natural brush, the next says use synthetic. Shellac has been around since the beginning of time what type of brush should be easy to answer in 2009.
The best brush for shellac depends on personal preferences. Some like a "mop" of softish natural hair like squirel that holds lots of shellac, but leaves no brush strokes as it flows out. Others like a low reservoir wash brush with very fine bristles that puts on finish very thinnly, also with no brush strokes. This can either be synthetic Taklon Gold or the much more expensive natural Kolinsky sable. Others prefer to pad on shellac with a cloth pad. The optimal pad is wool wrapped with fine linen (as in French polishing) or it is old well washed T-shirt folded up, or it is ....
In other words as with many things there is no "best", the answer is always "it depends." Try several things out and decide what you like. You won't be wrong.
The automotive paint books are good for specifics on two part finishes. There was a scary story where a guy said he was using two part finish ( second part is a hardener ) and he accidentally splashed a bit of hardener in his eye.
Before he could clean it out and call for aid his eye had hardened into a hard brown thing. Not sure where I read that. Could have been here in FWW several years ago.
It was not a haw hah the guy was serious.
roc
Give me six hours to chop down a tree and I will spend the first four sharpening the axe. Abraham Lincoln ( 54° shaves )
Steve,
There is no aspect of woodworking that is so full of double-speak and intentional obfuscation as finishing.
Thanks for that very clear explanation.
David Ring
http://www.touchwood.co.il/?lang=e&id=1
It's even more confusing than what you have described. Polyurethane isn't a specific substance, it's a chemical description. The poly part refers to polymers of which there are many, the urethane refers to the way it links with monomers, again, there are many. Even when you talk about more specifically about monomers, like diisocyanates, it's not a single substance, it's a family. Polyurethane can be used as a description for a wide variety of things from tires and foam to adhesives and varnishes. For the most part, this synthetic resin has good flexibility, resistance to solvents and abrasion. It's really about how molecules link at the atomic level. It's the linking that forms a film in varnishes. When you purchase something that contains "polyurethane" it just means it contains a man made resin that belongs to a chemical family.
Beat it to fit / Paint it to match
Absolutely. There are literally thousands of polyurethane varieties, depending on which polyols (in two classes, polyether and polyester) and which isocyanate (MDI, TDI, NDI HDI which divide into two classes, aromatic and aliphatic) and influenced by which catalyst is used, and whether link extenders and other additives are included in the process. All of these differences influence the properties of the finishes product. As far as single part uralkyd resins I don't think any manufacturer discloses anything about the nature of the polyurethane resin in the combination. In 2k products aimed at industry, more disclosures are made so the user does have to make choices. Certainly this choice goes well past my knowledge in the area. I am quite sure PhD candidates are, at this moment, writing dissertations on particular aspects of polyurethane chemistry.
I do gather that urethane and polyurethane are just synomyns with no functional difference separating products named by marketing departments or by authors making descriptions a bit more readable among a host of poly-**** chemical compounds.
What a great post. I agree with a heck of a lot of what has been said here. And I think the biggest point of contention is the fact the the companies that make the finishes we all use seem to be out to confuse the issue. They usually give us just enough information to get in trouble and one company's product isn't always compatible with another, or even a different batch number of their own.
Just like a lot of finishers, I will mix products, thin them, thicken them, and so on. I think that even though I am trying to find that magic bullet proof finish that will do it all, I have to except that it will always depend. It will depend on the wood .... the weather .... final use of the piece .... and sometimes my mood. All of these things effect how we work and how our work comes out. Like it or not, when we are finishing a piece, we really are artists at that time.
There was a time that I thought "Poly" finish was the best thing sense sliced bread .... Any more, I lean towards shellac and lacquer for most things.
But then again ....
It depends ;)
I guess I'm at the other end of all of this. I've tried the so-called "REAL" varnishes, tried shellac a few times. Always came back to Poly. It's something I know will work, and I can depend on. There IS a "learning curve" to using Poly, but once this is done, the results work for me. Note: It's a one-man, hobby-style, "shoe-string budget" shop.
Interestingly, there is absolutely no difference in using varnish with polyurethane and varnish without. They apply exactly the same way, cure in about the same time, and rub out quite similarly (though since non-poly resin varnishes are harder, rubbing out is a bit easier to achieve an easier sheen.)
I'm learning a lot on this thread. I appreciate your input.
when you speak of "mils" of film thickness , how does one, Me!! figure out how thick a finish is? On the other hand. how far does a thinned poly finish penetrate into wood such as birch?
Thinning should effect penetration but does it really mater? Or can it penetrate some wood like poplar and make it a little more water resistant?
Thanks, from one who thinks to much sometimes and not another.
Two ways. To measure the wet film, use a little "gadget", a gauge with graduated notches on the side. Insert into film, thickness is given by the last notch that the finish touches full height of the notch. To measure dry film thickness. Use micrometer to measure thickness of substrate before applying finish, re-measure when the finish has dried. I'll bet their are specific details about the substrate, etc, in some official standard, but I don't know that. Also, if you know the wet film thickness, and the percentage of solids you can calculate (pretty nearly) the dry film thickness of solids left after the volatiles have evaporated.
You do see industrial finishes where the technical data sheets specify the desired film thickness, both per coat over all.
This article shows a wet-mil gauge in use.
http://www.finishwiz.com/viscosityandwetmils.htmPicture is worth a thousand words, etc.
Thanks, that link makes it very clear how it works.
Shoemaker, 'mils' as a unit of measure is an interesting in that it only seems to be used regularly in North America. Here in Europe we mostly see the micron used to describe polish film thickness, and in many other applications. A micron, aka a micrometre, written ëm, is 1/1,000,000 of a metre.
Typically the recommended dry film thickness of pre-cat lacquer and other sprayed finishes is 150 ëm so that cracking problems don't occur. The manufacturers typically recommend applying three wet coats at 100 ëm each to achieve the desired 150 ëm dry film, or two wet coats at about 125- 150 ëm each.
To give you an idea of what 150 ëm looks like it's about 0.006" or 0.15 mm. A human head hair is 100 ëm in diameter.
North American producers and suppliers of pre-cat lacquers, post-cat lacquers, etc, generally recommend a wet film application at about 4 mils, and to apply three coats. Alternatively they sometimes suggest applying two wet coats at about 5-6 mils thick each.
There are 25.4 ëm per mils, therefore 4 mils = ~100 ëm = one average human head hair.
So, in a long-winded way you can tell you've got about the right thickness of wet finish application by dropping a head hair into the wet film immediately after you've applied it. This is nowhere near as useful as the little comb like device byhammerandhand linked you to a couple of messages before this one at this link, http://forums.taunton.com/fw-knots/messages?msg=47006.20
As you can appreciate, using the head hair method leaves you with problems such as:
removing the disfiguring hair from the dried film
assumes you've got head hair
assumes your head hair is about average thickness
I think you'll realise there's meant to be a certain amount of humour in this post, as well as some serious information. Slainte.richardjonesfurniture.com
Pardon the rant. ?? All words well said Sir...
No rant at all.. Your thoughts (rants?) should have been four pages long each over several days...
I usually use a 'wipe on Poly' after finishing.. Minwax satin. I do NOT like gloss on any wood. I go for a smooth finish to the touch. I hardly ever stain. (Not that I NEVER use a stain) I usually use a wipe on oil varnish first and let it color the wood as it will. I just try to bring out the woods beauty without trying to change it.. Sort of like my wife did.. She just used a bit of eye makeup.. All she needed to please herself and maybe me?.
That is not to say not to use Gloss if YOU like it..
I apply one 'wet' coat of Poly, well applied and then rubbed off as it dries. That one coat may be several applications of wet as it blends in and dries off.. Then I use wax and elbow grease applied with the gray 3M pads to get off any nubs..... Followed by a car buffer with a sheeps wool cover..
I hate finishing but usually turns out OK.. 'As I like wood to be' Not that I am making something for a woodworkers show though.. Just friends and family things...
And I WILL use wood with sap wood exposed. In fact I look for it if I can cut to size (or adjust the project to fit it in). And wild grain.. I look for it too!
Edited 6/15/2009 5:28 pm by WillGeorge
Steve - I'm doing some digging and maybe you can help. I found the following in a 2008 thread about using ployurethane over shellac, by Rob A
"When you start putting different finishing materials on top of each other is when you start to open yourself up to real trouble. For instance polyurethane (oil based or waterborne) on top of shellac that still contains wax is asking for problems with the poly flaking off. Same with any waterborne finish over waxy shellac. Shellac and laquer don't mix very well either. The solvent in laquer can cause shellac to become gummy and soft."
I am experimenting with using shellac (Zinzar Clear) as a sealer under wood filler on white oak, over a dye stain. I following a suggestion in a FWW article. I am lightly sanding the wood fillier residue, prior to applying a topcoat. I am leaning toward wipe on varnish for durability. I am reading about the reaction between the shellac and varnish, and now I am wondering about my top coat options. I am not hard over on a built up finish, and I wonder if tung oil, or another oil, will work? Any suggestions are appreciated.
Zinsser Clear is a shellac that contains wax. Polyurethane varnishes including wiping varnishes may not adhere well. As Rob was saying, you only want DEWAXED shellac under polyurethane or any waterborne finish. The only dewaxed liquid shellac made by Zinsser is Seal Coat. If in doubt use SealCoat (or mix your own from dewaxed flakes) when you may want to put another material over it.
If you still have a choice use dewaxed under the filler, though since the filler doesn't have a poly based binder, you should be OK using it over the Clear. You won't have a problem with a wiping varnish over the filler IF you use a wiping varnish that does not contain polyurethane. Traditional resin varnishes have more adhesiive power in these situations. Oil finishes--oil/varnish mixes or pure oils, done work well over fillers.
Steve- thanks! I will use the dewaxed Shellac, and wipe on varnish. BTW, for the first time, I use alkyde dye, and got excellent color. Will the color hold up well over time with the varnish?
Aniline dye (which is what I am pretty sure you meant to type), may fade over time if it is in lots of sun, but it will do so much less due to the stain effect of the filler. Varnish may slow the change a bit. But the problem won't be major for most interior uses. The wood itself will be undergoing changes itself. All of this will be going on at rates slow enough that it is unlikely to create a concern.
analyne die it is ! Thanks for all the advice.
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