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Varnish vs Polyurethane vs no idea

rdreid's picture

Okay so I know this has to be posted in hear, and unfortunately may fall in line with the "dumbing down" that other people have been frustrated with, but, alas I am so confused I have to ask.

I do not understand the difference between a varnish and a polyurethane.  I seem to see the words used interchangably and I thought they meant the same thing.  But then I was looking and saw stuff about spar varnish, oil-varnish, and some really confusing article on polyurethane finishes.  (then there is the whole lacquer thing which I don't get either...but since I have no spray stuff I just figure ignorance is bliss).  I read about Garrett Hacks homemade varnish with boiled linseed oil, turpentine, and varnish?!? 

Here are my questions in a nutshell:

When someone says they used varnish as their finishing coat, are we talking about the can of minwax polyurethane that I have sitting in my garage or is there something else called varnish? (I know there are other brands but that was just my example)

What is the difference between polyurethane and varnish if there is one...both in chemical make-up and pros and cons of using it.

Does the spar varnish finish Lon Schleining used in his article in 2002, have the same protection as a polyurethane?

Finally the whole solvent thing was cleared up after an article I read...but I still have the question does thinning something make it have a less hard finish or does it affect the quality of the finish at all?  Is it just used to affect dry time?

Lost in fumes,



dwolsten's picture

(post #121090, reply #1 of 11)

Your questions would take a whole book to answer completely. For that, I refer you to Bob Flexner's book about finishing. It'll answer all these questions and more.

The simple answer is this: varnish is a material which consists of resin and solvent. You put it on wood, the solvent evaporates, and the resin is left. Polyurethane (the Minwax at Lowe's) is a type of varnish, although many people seem to be very confused about this. There's three kinds of resins used in varnishes for woodworking: phenolic, alkyd, and polyurethane. Polyurethane has become the most popular these days, mainly because of its greater scratch resistance. Some people complain it looks cloudy, however, although I think this is usually because they use too much.

Varnishes are either water-based, or oil-based, much like paints. Oil-based are older, and probably use either boiled linseed oil or tung oil as their base. They usually impart an amber coloring, because of this oil content (esp. with the linseed oil). Water-based varnishes don't have the amber color, though some say they're not as durable, and other say they've improved with time and the newer ones are.

You can buy non-polyurethane varnishes, but probably not at Lowe's or Home Despot. Some people prefer the phenolic because it's extremely hard and can be polished well. The alkyd preceded the polyurethanes, although typical polyurethanes I believe also have alkyd resin, and are not 100% polyurethane.

Thinning a varnish I think is usually done for the same reason as with paint: to lay thinner coats, in order to avoid drips and runs. "Wiping varnish" is an example of this: the wiping polyurethane you can buy at Lowe's by Minwax is really just regular polyurethane varnish thinned with 50% or so mineral spirits. I've made my own like this, and I greatly prefer it to brushed-on poly because I don't have all the problems with drips and runs I had before. However, it takes many more coats to get the same protection.

SteveSchoene's picture

(post #121090, reply #4 of 11)

It's not true that varnish is just resin and solvent, or that it dries by solvent evaporation.  Varnishes cure with a chemical reaction that takes place as and after the solvent evaporates.  When it has cured, the mineral spirits that are the solvent in oil-based varnishes cannot dissolve the varnish again. 

Shellac and lacquer do dry by evaporation--and the same solvent can redissolve them again.  Shellac was at one time termed a "spirit varnish" but that isn't current usage of the term varnish. 

Test your finish on scrap, FIRST, or risk having to scrap your finish.

dwolsten's picture

(post #121090, reply #5 of 11)

Yes, you're right. I should probably read my book a little more carefully.

rdreid's picture

(post #121090, reply #6 of 11)

Thank you so much to all of you...I think I am understanding it a little better.  I am one of those people that needs to see the whole big picture before the steps make sense to me.  I was getting parts of the varnish info but not seeing the whole thing.  All of your responses really helped.

Now that I understand that, I have a question about, polyurethane application.  I have heard some people use brushes and some use foam applicators and some just wipe it on.  All of those seem to have their own merit and boil down to simple preferences.  My question is what is done after it is applied.

I have heard that you should not wait too long after each application because of the chemical reaction that takes place as the varnish evaporates.  Is it true that if the layer has cured too much you will not get a good adhesion with another coat?

Also, what is rubbing out a finish?  Is it done with polyurethane and if so how?  I have read a bunch on using different waxes, sandpapers, etc but I don't know that I've ever read one that I know is dealing with polyurethane.

Thanks again guys...the varnish thing was really a frustrating thing for me and a cause of a great deal of confusion while I am trying to get started with a hobby that I am really beginning to enjoy.

SteveSchoene's picture

(post #121090, reply #7 of 11)

Polyurethane varnish sometimes has a "window" where it is dried enough to have another coat applied, but not so fully cured that the first coat needs sanding to adhere.  If this is the case with a particular product it should say so on the label.  If you wait past the window all you need to do is a light sanding with 320 grit sand paper to provide enough "tooth" for the second coat to adhere.  Since a scuff sanding isn't a bad idea anyway to keep the dust nibs and other "artifacts" at bay it's not very important to stay within the "window". 

Varnish almost always benefits from rubbing out, since unless you have a "clean room" there is almost always some dust needing to be removed.  Rubbing out consumer grade polyurethane varnish is difficult because it is softer than the traditional resin varnishes.  As a consequence it is harder to get a really even sheen on poly.  That's one reason I don't use poly.  Traditional resin varnish (but not spar varnish) does better.  The champions in easy of rubbing out are shellac and nitrocellulose lacquer. 

The first step toward successful rubbing out is to give it a month to cure before beginning the rubbing out.  The first step in rubbing out is to sand out the defects--dust and other problems. You need to be careful not to cut through the final coat, or you will see "witness rings" that appear as hazy contour lines where the cut through as occurred.  I usually start with 600 grit--occasionally 400 if it has lots of problems.  I then sand up to 1000 if I intend a satin finish, or to about 1500 grit if I will go on to gloss.  After the sanding, I use rottenstone on felt, lubricated with paraffin oil.  Fine rubbing compound will work too. 


Test your finish on scrap, FIRST, or risk having to scrap your finish.

rdreid's picture

(post #121090, reply #8 of 11)

Thanks...all of you have spent so much time writing up answers.  I appreciate it.  One thing I can say for the woodworking community is their general desire to help out people beneath their skill level.  Very cool to see and very much appreciated, especially as someone on the bottom of the skill ladder...


ptu's picture

(post #121090, reply #9 of 11)

When I did the interior of my house in poplar and birch, I used both water based urethane(Diamond coat) and oil based(polyurathane). The oil based brought out the grain structure very nicely, but the water based was very flat looking. for looks I prefer the oil based, but it did darken the wood slightly. I see that most people say that the oil base is harder, but we prefer the water base for toughness. We use it on floors with superior results. On my birch stairs, I used oil base to bring out the grain, and after it was very dry, covered it with water based. I am very satisfied with the toughness, and if done right, works well. A word of caution, the oil base must be dry or it will lift. This doesn't explaine the differences of varnishes, but will give an idea of some applications. I imagine that some people will twitch over my warer over oil, but done right, it give good results.

HowardAcheson's picture

(post #121090, reply #10 of 11)

The reason that oil based varnish "popped" the grain is that the oil gives it an amber or yellow cast. It's the amber color that "pops" the grain. Waterborne finishes are generally water clear (no amber color), so they do not "pop" the grain. You can get the same effect if you add a little waterbased dye to the waterborne acrylic. Now, it too, will be amber and will "pop" the grain.

What benefit do you feel you get by using an oil based undercoat before applying the waterborne acrylic?

ptu's picture

(post #121090, reply #11 of 11)

Not knowing how to pop the grain other ways. The Diamond water base is such a good finish that I wanted it for ware on my stairs and bedroom floor.

SteveSchoene's picture

(post #121090, reply #2 of 11)

There is polyurethane varnish, alkyd varnish and penolic varnish as well as some exotics from the violin making world, and special purpose industrial products.  All are equally varnish, which now means an oil chemically reacted with the resins to create a new compound.  Single part polyurethane varnishes in the consumer market place aren't pure polyurethane resin--they typically have alkyd resin as well.  Varnishes have different properties depending upon how they were formulated.  The resins named are just different plastics that provide different properties when used to manufacture varnish. 

Spar varnish (sometimes called "long oil" varnish) is made with relatively large proportions of oil to resin in the ingredients.  This makes it flexible for exterior use.  For marine use, where sunlight (UV) can damage the wood and the finish, UV absorber chemicals are added to spar varnish to slow the UV damage.   

There are plenty of products that are truely varnish that don't have polyurethane as an ingredient.  Behlen Rockhard,  Pratt & Lambert 38, McCloskey Heirloom, Ronan Quick Rubbing Varnish

Garrett Hack doesn't have "homemade varnish" he has a mixture of varnish and oil, with turpentine as a thinner.  If there is a relatively large amount of varnish in the mix, it's properties will be more varnish like.  But adding oil outside of the chemical reaction doesn't make a regular varnish into "spar" varnish, its still a mix of oil and varnish, not a new kind of varnish. 

As far as protection in home situations, single part polyurethane's largest pro is abrasion resistence.  It's cons include being a little less clear, and being more susceptible to UV damage.  The toughness that goes along with abrasion resistence also makes the typical polyurethane varnish more difficult to rub out evenly.  Polyurethane varnish (again the single part consumer products) also has adhesion problems.  For example, it won't stick well to shellac containing wax. 

For typical oil-based varnish adding thinner doesn't affect the end result.  The "drying faster" is mostly, but not entirely illusion, as the thinner evaporates.  The curing is a chemical reaction that is basically the same without the mineral spirits, though the reaction with air may be speeded up given the thinner coats that result.  The paint thinner just evaporates away and doesn't participate in the final finish.  But, if you thin 50-50 to make a wipe on varnish, it probably takes 3 or even 4 coats to build the same thickness as a coat of "full strength" varnish applied with a brush, so it is easy to appear to see lots of difference. (Different story with waterborne--add too much water and all bets are off.)   

Anyway, protection isn't the only thing to look for when chosing a finish.  How it looks, how easily it applies, and how easily it can be repaired if damaged, are all quite important.  In my opinion, it is the marketing hype that has pushed "poly" into dominence in the consumer market.  Polyurethane varnish is great for floors, and in industrial appliations, but overkill for most furniture applications.   

Don't know what Lon Schleining's finish was so can't comment. 

Test your finish on scrap, FIRST, or risk having to scrap your finish.

HowardAcheson's picture

(post #121090, reply #3 of 11)

Let's start at the beginning. Varnish is a mixture of a resin and a drying oil that is mixed together and then heated until it becomes a new compound called varnish. Thinners are then added to make it easy to brush.

There are a number of resins used. They used to be natural resins but in the past 75 years synthetic plastic resins have been used. These include phrenolic resin and, most often alkyd resin. Since the 1950's some urethane resin has be added to the other resins or urethane resin used exclusively. The urethane increases the scratch and heat resistance of the varnish..When urethane resin is used the manufacturers came up with the name "poly" or more correctly "polyurethane varnish". But poly resin varnish is just varnish made with a poly resin. Polyurethane is cheaper than the other resins so it has become more marketed than the non-poly resins.

There are also a number of different drying oils used. Linseed oil and tung oil common but the most used is a vegetable oil. Vegetable oils are the lightest in color. Tung oil is lighter than linseed oil but it is still darker than vegetable oils.

Manufacturers vary the amount and type of both the resin and the oil to impart particular performance characteristics to their product. For exterior varnishes, more oil is added to make the varnish softer and more flexible. A dollop of UV inhibitor is also added to protect the finish. Exterior or marine varnish is no more water or watervapor resistant than interior varnish. Interior varnish is harder and more protective so should always be the varnish of choice for furniture used indoors.

So, to answer your question, varnish is varnish. Poly is just a type of varnish. It is tougher than non-poly varnish but tends to be cloudy or it is not as clear. Non-poly varnish will give a nicer appearence.

>>When someone says they used varnish as their finishing coat, are we talking about the can of minwax polyurethane that I have sitting in my garage or is there something else called varnish?

The could be talking about a poly varnish or a non-poly varnish. Probably they mean the poly varnish.

>> What is the difference between polyurethane and varnish if there is one...both in chemical make-up and pros and cons of using it.

Explained above, I think. Non-poly is clearer, poly varnish is more scratch and heat resistant.

>> Does the spar varnish finish Lon Schleining used in his article in 2002, have the same protection as a polyurethane?

Spar varnish in most cases is just a marketing term for a manufacturer's exterior finish. True marine spar varnishes are used on wooden masts where the finish needs to be flexible and soft to bend with the mast. It's not a finish that is very tough. Also, polyurethane varnishes should not be used where they are exposed to the sun. UV rapidly deteriorates poly varnishes. They yellow, crack and peel very quickly. Minwax Helmsman for example, was the first finish to fail in Consumer Reports long term tests of exterior clear finishes.

>> does thinning something make it have a less hard finish or does it affect the quality of the finish at all? Is it just used to affect dry time?

Thinning has no affect on the performance of the varnish except as to how it brushes. When the thinner evaporates you are left with the varnish being the same as if it wax never thinned. Thinning has little or no affect on dry time in general. Oil based finishes "dry" in two steps. First the thinner evaporates and the surface becomes tack free. The second stage starts then which is an oxidation process where oxygen reacts with the varnish causing the varnish to cure to a hard finish throughout the film. This curing takes 3-4 weeks for the finish to fully cure an develope full adhesive strength and protective qualities.