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Thread: Poly? Urethane? Polyurethane?

  1. #1

    Poly? Urethane? Polyurethane?

    I'm pretty ignorant about finishing products, but there seems to be a general consensus that "poly" finishes aren't as nice as more traditional shellac/varnish/lacquer finishes. (With certain exceptions, such as high wear or moisture applications.) As I understand it, the poly finishes aren't quite as rich; they're more artificial looking.

    Meanwhile, I've recently started using General Finishes' Arm-R-Seal, which the label calls "oil and urethane". I like the stuff, and see quite a few others here use it too. Which leads to my questions...

    Is there a difference between urethane and polyurethane? If so, will I understand the difference without a degree in chemistry? Or is the difference of any real concern? Is Arm-R-Seal really a polyurethane, labeled in such a way as to make me think it's somehow better? Will my friends still respect me in the morning* for using a urethane finish, whereas they might not for using a polyurethane? Please edumacate me.

    Thanks -

    - Vaughn

    * OK, we all know they don't respect me anyway, but figuratively speaking...

  2. #2
    Join Date
    Aug 2004
    Baltimore, Md

    Poly in this sense is a prefix to Urethane, so polymers are added to urethane (basically a rubber) to form longer chain molecules or polyurethane. The short answer I believe is no, urethane by itself is not a polymer based finish. So you are not using an polyurthane.
    "The element of competition has never worried me, because from the start, I suppose I realized wood contains so much inspiration and beauty and rhythm that if used properly it would result in an individual and unique object." - James Krenov

    What you do speaks so loud, I cannot hear what you say. -R. W. Emerson

  3. #3
    Join Date
    Dec 2004
    Santa Rosa, CA
    Vaughn, it's really easy to be confused by the terms used in finishing. Seems like there is no 'truth in labeling' concept here. Lots of terms tossed about with no consistancy.

    Quoting from Jeff Jewitt's 'Great Wood Finishes' - "Varnish refers to finishes made from hard, durable synthetic resins that are modified with a drying oil. The resins have names like alkyd, phenolic and urethane, and the oils are tung and linseed. ... Urethane varnishes are generically referred to a polyurethane and have the best resistance to heat, solvents and scratches."

  4. #4
    Join Date
    May 2005
    Moore, SC

    Good finish - but just thinned polyurethane.


    I too have been using ARM-R-SEAL and I think it is a pretty good finish, and really easy for me to apply. As I understand things it is simply a wiping polyurethane, which is simply a regular polyurethane that has been thinned down with mineral spirits, or some other thinner.

    If you want to read a good article about this, check out this link.

    near the bottom you can download a free copy of this magazine, and it contains an article titled "Understanding Wipe on Finishes". Also there is a good article in the november popular woodworking magazine about this.

    I dont think it makes you less of a woodworker, and I saw that box you posted a few days ago and it was awesome.

    I personally think the key to any finish is how well you use it and execute it. We can debate which is best for durability, water resistance, historical accuracy, etc......but I think all that takes second place to what the finish looks like. You are getting some great results with the ARM-R-SEAL on your projects and I wouldnt worry one bit about what it really is.


  5. #5
    Join Date
    Feb 2003
    Shoreline, CT
    I'm one of those who argues in favor of natural resin varnishes instead of poly in most furniture situations. The difference in appearance is greatest for finishes that build a significant film thickness. On thin, in the wood, finishes or on very thin film finishes the appearance difference is not terribly significant. If you like the looks, then its OK.

    Of course, thin coatings give up the protective properties of urethane/polyurethane/poly so there isn't really much benefit in using the poly. These very thin finishes aren't appropriate for much rubbing out either. Consequently, the fact that poly is a bit harder to rub out well doesn't enter into the equation either.

    In film thicknesses used to provide protection, its my personal opinion that traditional resin varnishes offer sufficient protection from abrasion, water, and other chemicals to be used for such things as table tops. It is in these thicker films where the bit of cloudiness in poly becomes apparent. Poly is tougher, but if a more attractive finish is fully satisfactory in its toughness, then there isn't much reason for poly. Others may make the appearance/toughness trade off differently. Similarly, the ease of rubbing out the finish is of more importance to me than to others.

    By the way, we should all be incensed that finish manufacturers are so close- mouthed about the contents of their products. Its a ruse, mostly designed to make us think there are large difference among products, when the reality is they are really pretty much the same within broad classes.

  6. #6
    Join Date
    Mar 2003
    Southport, NC
    >> Arm-R-Seal, which the label calls "oil and urethane"

    Just another way of saying "thinned polyurethane varnish".

    As has been said oil based varnish is a resin and an oil mixed together and heated. At a certain temperature the mixture combines into a new compound called varnish. All current varnish resins are plastics of some type. Phenolic, alkyd and urethane (polyurethane) are the plastic resins used. The oils are linseed oil, tung oil and/or one of a couple of semi-drying oils. So as far as the basic product, all varnishes are pretty similar. What makes for some slight differences is the type of resin and oil and the proportion of oil to resin. Phenolic resin and tung oil makes for a slightly more water resistant finish and these varnishes are the true marine products. Add more oil and the finish becomes more flexible and soft so that it can be used on softer wood or where the finish must be able to flex. Softer finishes are naturally more scratch resistance and almost all polyurethane varnishes are relatively soft.

    Urethane resin is less expensive than the others and, combined with it's somewhat greater scratch resistance and heat resistance, it has taken over the varnish market. There are no hard and fast rules with finishes but, to a great extent, the thicker you build the finish, the "plasticy" it will look. It makes little difference whether you use a urethane resin product or one of the others, they all make the wood look like it were encapsulated in plastic. But, urethane gets a cloudy appearence when it gets thick and many folks refer to this as the "plasticy" look. Phenolic and alkyd resins are clearer.

    I prefer non-urethane finishes in general. But, there are certain applications where the additional toughness of urethane is a benefit. For kitchen tables that get hard use, hall tables that get the car keys thrown are where a urethane finish might be the one to use.

    Finally, 2-3 coats of full strength finish is just as durable as more coats. Scratches occur in the first couple of millemeters of the finish whether it is one coat thick or six coats thick. 2-3 coats get you to the thickness where water and watervapor protection is maximized so there is little benefit to applying more coats. Remember, the more coats, the more likely any finish will look "plasticy".

  7. #7
    Thanks for the info guys. I was hoping to get a few of the "finish gurus" to chime in, and I'm glad to see they did. I've learned a lot in just a few posts. I see my suspicions about Arm-R-Seal were correct, but I agree with Andy's point -- what really matters is how it looks. And in my case, even the best finishing product can look bad if improperly applied. DAMHIKT.

    Thanks again -

    - Vaughn

  8. I'm about to embark on my most ambitious project yet, and I plan on spraying it with my Wagner Fine Spray HVLP gun, so I too am looking for info on finishing as well.

    I'll jump in here and give you my two yen's worth, but please understand, that this does not come from my wealth of experience with finishing, just research I've done and a lot of reading.

    The book that I've found of most use is "Understanding Wood Finishes by Bob Flexner". Certainly worth the price of purchase, and better if you can check it out of your local library.

    I've been able to decipher the badly labeled cans of finish I have, to figure out what they actually contain (add to that the Japanese to English problem), and I've now got a handle of the theory of how finishes work. What I need now is to make some more stuff, so I can apply (literally) this knowledge<!--[if gte vml 1]><v:shapetype id="_x0000_t75" coordsize="21600,21600" o:spt="75" oreferrelative="t" path="m@4@5l@4@11@9@11@9@5xe" filled="f" stroked="f"> <v:stroke joinstyle="miter"/> <v:formulas> <v:f eqn="if lineDrawn pixelLineWidth 0"/> <v:f eqn="sum @0 1 0"/> <v:f eqn="sum 0 0 @1"/> <v:f eqn="prod @2 1 2"/> <v:f eqn="prod @3 21600 pixelWidth"/> <v:f eqn="prod @3 21600 pixelHeight"/> <v:f eqn="sum @0 0 1"/> <v:f eqn="prod @6 1 2"/> <v:f eqn="prod @7 21600 pixelWidth"/> <v:f eqn="sum @8 21600 0"/> <v:f eqn="prod @7 21600 pixelHeight"/> <v:f eqn="sum @10 21600 0"/> </v:formulas> <vath o:extrusionok="f" gradientshapeok="t" o:connecttype="rect"/> <o:lock v:ext="edit" aspectratio="t"/> </v:shapetype><v:shape id="_x0000_i1025" type="#_x0000_t75" alt="" style='width:12pt; height:12pt'> <v:imagedata src="file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msoclip1/01/clip_image001.gif" o:href=""/> </v:shape><![endif]--><!--[if !vml]--><!--[endif]-->

    Basically there are two kinds of finishes;

    Penetrating This type is a straight oil and cures soft, so it should not be "built up" on surface of the wood.

    finishes cure hard, so they can be built up to whatever thickness you want (within reason). There are five kinds of "Film" finishes

    Varnish (polyurethane is a type of Varnish)
    Water Base
    Conversion (conversion varnish and catalyzed lacquer)

    The most important difference that I have learned about these film finishes are how they cure, there are three methods;

    (Solvents are alcohol, acetone, and lacquer thinner)
    Pigment stains with Lacquer binder
    Wood Putty with Lacquer binder
    Wax (Solvent is mineral spirits or turpentine)

    (Thinners are mineral spirits and naphtha, often listed as "petroleum distillates")
    Linseed oil and Tung oil
    Oil/Varnish blend
    Wiping Varnish
    Pigment stains with oil or varnish binder
    all in one stain, seal, and finish suing an oil or varnish binder
    Paste wood filler with oil or varnish binder
    Glaze with an oil or varnish binder

    (Solvent is glycol ether; thinner is water)
    Water based finish
    Pigment stains with water based binder
    All-in-one stain, seal, and finish using a water based binder
    paste wood filler with water based binder
    Glaze with water based binder

    Shellac and Lacquer are evaporative finishes that cure by the evaporation of the solvents in them.

    Varnish, curing oil and conversion finishes are reactive, they cure by a chemical reaction, taking place, after most of the thinner has evaporated. Conversion finish cures when a catalyst is added.

    Water based finish is a coalescing finish, it is made up of already cured chunks of finish that are suspended in water, it cures when the water evaporates, and the chunks then coalesce into the film.

    Ok more on each one.

    Evaporative finishes are made up of solids that are dissolved in a solvent. When the solvent evaporates, the solids are left, and they form a film finish on the wood. The can be imagined as tiny long skinny strands of solids, when the film forms, the long strands intertwine to form a fairly hard surface, but the strands of solids, while interlocked do not bond to each other. This type of finish can be re-evaporated by introducing the solvent suspending the solids again.

    When you apply a second coat of the evaporative finish, the solvent in the new coat will partially re-dissolve the first coat. Evaporative finishes cure from the bottom up.

    Reactive finishes change chemically when they cure. As the thinner evaporates the resin chunks come closer together, then the chemical reaction happens and they link on a molecular level in a network like those tinker toys we played with as kids. This is often called "Cross-linking or Polymerization". When you apply a second coat of this kind of finish, the first coat is not affected; it does not soften at all. Oxygen is the catalyst in most. The pros use conversion finishes which have catalyst added to them like 5-minute epoxy, but they are used only by the pros.
    When you want to apply a second coat of reactive finish, you have to scuff the finish to create a mechanical bond between coats. As reactive finishes cure from the top down, and uses oxygen to do so, thin coats are a must. Applying a fresh coat of reactive finish over a coat that is not totally cured with often result in a nice wrinkle overnight, as the underlying coat had not finished releasing its Oxygen.

    Coalescing finishes are usually water based, and have a bit more going on that either reactive or evaporative finishes, and are actually a combo of the two. This coalescing finish is made up of chunks of already cured, cross-linked finish suspended in the water. There is also a solvent, usually glycol ether, which evaporates slower than the water does. This solvent softens the chunks of already cross-linked finish, and as the water evaporates, the softened chunks come close together, and connect, or interlock with each other, but they do not cross-link with each other. This type of finish also cures from the bottom up.

    The main diff between the three types is how they cure, and how the molecules cross-link in the finish. The only finish that really cross-links is the reactive finish, it forms a very hard to break apart finish and is very tough. The evaporative finish does not cross-link at all, and can be re-disolved at any time by the use of it's solvent.

    The evaporative finish may not be as tough, but that has some advantages, it is the easiest finish to rub out to a nice sheen, to repair, and to strip, while the super tough reactive finish has it bad points too, it is hard to rub out, repair or strip. The coalescing finishes are in a middle ground between the two, they have cross-linked chunks that are the main part of the surface, but they can be softened by a thinner because the chunks are not cross linked to each other, but only interwoven.
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>
    OK great, nice info, but now what do we do with it?
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>
    I guess you have to decide what you want.
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>
    For me I really have only two choices, the Coalescing finishes (water based) or the Reactive Finishes (Varnish Polyurethane) as Iíve never seen the Lacquer in anything bigger than a about a quart sized bottle, and it was expensive!!
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>
    I have done my best to get rid of the dust in my Dungeon, but really, I cannot get away from it, so the reactive, finishes, the varnish and polyurethane are hard to use, the water based finishes seem to be the best bet, and I hear that spraying them with the turbine type HVLP unit I have they dry really quickly, as the air the turbine produces is fairly warm. I understand that on of the biggest beefs against the water based finishes is on darker woods, they give the dark woods a bland washed out look, well the ash Iím using should be OK then.
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>
    Iíll post pics when Iím done, and we shall see how well all this book work had stood me.
    <!--[if !supportEmptyParas]--> <!--[endif]--><o =""></o>

  9. #9
    Join Date
    Feb 2003
    Shoreline, CT
    Quote Originally Posted by Howard Acheson

    Finally, 2-3 coats of full strength finish is just as durable as more coats. Scratches occur in the first couple of millemeters of the finish whether it is one coat thick or six coats thick. 2-3 coats get you to the thickness where water and watervapor protection is maximized so there is little benefit to applying more coats. Remember, the more coats, the more likely any finish will look "plasticy".
    Its not really true that the maximum watervapor protection is reached after three coats. The data look close to being that way, but the most commonly reported figures are based on one days exposure to 90% relative humidity. Then, using gloss polyurethane varnish shows moisture exclusion effectiveness, MEE, of 89, and six coats only increases to 93. However, when the test is longer, the effect of more coats becomes more significant. For two weeks, MEE with three coats is 44 and with six coats is 62, and increase of over 40%. In general, the impact of larger number of coats is greater with non-pigmented finishes. These data are from a Forest Product Labs study. The full study is on-line at

    This is undoubtedly why makers of marine varnishes specify at least 5 or 6 coats, or a total film thickness of 10 mils which is about 250 microns. By the way even this thick coating is considerably less than one millimeter. But three coats of varnish will only reach about 4.5 mils total dry film thickness, which would be considerably lessened if the finish were sanded between coats. Three coats of full strength varnish, brushed on finish is roughly nine coats of wiped on varnish, depending on how thin, and how applied.

  10. #10
    Wow Stu, that's a lot of information to digest, but it makes sense to this dummy. Thanks much.

    - Vaughn

  11. Quote Originally Posted by Vaughn McMillan
    Wow Stu, that's a lot of information to digest, but it makes sense to this dummy. Thanks much.

    - Vaughn
    You and me both dude!

    I have to admit, I've read this stuff several times, and still I go back to look stuff up, just to be sure

    Having to explain it to someone else really helps this dummy explain it better as well.

    The Flexner book is very well done, you can skim it and get what you need to finish what you are working on, or you can go deeper and come away with a much better understanding, which is what I'm trying to do.


  12. #12
    Hey Stu, great tutorial on the chemistry of finishes!


  13. #13
    Join Date
    Mar 2003
    Southport, NC
    Hi Steve,

    Yes, I'm familier with the tables in the Wood Handbook but let me say that for furniture, it is an unusual circumstance where additional water and water vapor protection is required. No matter what the thickness, moisture will get through during the annual seasonal relative humidity changes. It makes little practical difference whether it impedes moisture for three days or ten days.

    >> This is undoubtedly why makers of marine varnishes specify at least 5 or 6 coats,

    I learned a good portion of my finishing in the marine environment. The primary reason for 5-6 coats is for UV protection. It takes a certain film thickness to provide the UV protection and to allow the sandoff of some of the finish when refinishing. It's true that even marine interiors will get more coats than furniture, but is because of the much more pervasiveness of moisture in marine applications. How often do you hose down a dining room table.

    That said, the more film thickness built up when using a fairly thick product like varnish, the more "encased" the surface will look. Some like that look and that's fine. It's not my personal choice but if someone wants it, I'll do it. With some finishes like lacquer and shellac, too thick a finish can be detrimental due to the brittleness of the finish.

  14. #14
    Join Date
    Feb 2003
    Griswold Connecticut

    Arm-A-Seal is a "wiping varnish". It is everything that has been stated here. With one exception. It's a darn nice product. Easy to use, almost foolproof. If the flood coat is done with the Seal-A-Cell it works out even better.
    One downside is that it has no UV protection that I'm aware of.
    The book that Stu refered to, is a must for any woodworking library. It really breaks down the "mystique" of wood finishes into " simple english".
    Great book.

  15. #15
    Join Date
    Feb 2003
    Shoreline, CT

    Of course UV is the primary reason for the thicker marine coatings and they are absolutely necessary in that context. I should have stated that differently.

    And, while I would tend to use 4 or 5 coats of varnish on a kitchen table top, by the time the first several of those have been used to complete any grain filling, and elimination of the artifacts associated with not having a "clean room" I doubt I end up with the equivalent of more than 2 or 3 full, unsanded coats. In most cases, you are correct that that is fully sufficient. Such a finish will still significantly reduce the impact of seasonal moisture swings.

    But it is still important in choosing a finish to weigh the fact that more film thickness really does imply more protection, and that a very thin film--three coats of a thinned wiping varnish, for example--doesn't convey the protection that one might think is automatic with "varnish". This has to be balanced with appearance. It's a trade off, not an absolute choice. No free lunch.

    As far as poly goes, I don't use it on furniture because if I am using varnish at all, instead of much more attractive shellac or other spirit varnishes, it is because I want the protection needed for a kitchen table or a plant stand. Since this entire choice is predicated by the need for protection, and I know I have to have enough film thickness to do that, then I will not be willing to give up the clarity of the alkyd or phenolic resins, too.

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