kmealy

TGIF: Varnish 101 June 20, 2017

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Varnish 101

Let’s explore one of the most popular finishes – oil-based varnish

What is varnish?

Varnish is made by cooking a resin with oil yielding a new substance, varnish.   Originally the resins were natural products (think Stradivarius) but beginning in the early 20th century, synthetic resins started to be used.  And yes, Polyurethane is just one common type of varnish, using urethane (and alkyd) as the base resins. Varnishes are made by selecting 1 or 2 resins from column A and one oil from column B

Resin

Oil

Sample products

Urethane – toughest, but can be cloudy and has minimal UV resistance,  doesn’t bond well to other finishes or to its cured self.

Linseed

 

Products that say polyurethane – Minwax, Varathane, Sherwin-Williams Polyurethane

Phenonic  - tough and flexible, but darkens

Tung – often used with phenolic

 

Waterlox

Alkyd – less tough, but lighter color

 

Soy (soya) – less yellowing

 

Pratt & Lambert #38, Cabot 8000, Sherwin –Williams Fast Dry (usually products that don't say "Tung Oil" (phenolic) or (poly)urethane on the can.)

 

The choice of which used impart certain characteristics to the final product, just as the choice of flour(s) impart certain characteristics to bread.

Once cooked, the varnish is thinned with a carrier, usually mineral spirits, to make a workable product.  Also added are driers to speed curing and unless it’s gloss, flattening agents (silica).

How Varnishes Cure

Varnishes “dry”, or cure, in two phases

1.    The thinner (e.g., mineral spirts) evaporates.  Known as “flashing off.”

2.     The varnish absorbs oxygen and  polymerizes into long chains.  This can take about three weeks to fully cure.  It helps to be at moderate temperature and fresh air.   (This is also why it cures in a partial can.)  Flexner calls varnish finishes "Tinker Toys" because of the way they cure.

Once the product cures, it will not “re-dissolve” in the thinner, mineral spirits.

 

Characteristics of varnish vs. other finishes

Pros

Cons

Readily available. You can find polys at grocery and discount stores.

Non-polys harder to find outside specialty stores

Ability to apply with simple equipment.   Long open time allows time to brush /pad out.

Long curing time, increasing finish time and picking up dust.  Cooler temperature increases cure time.  Long dry time makes it problematic to spray.

Durable finish, resistant to water and water vapor, heat, many solvents

 

Difficulty to repair or strip (the converse of durability)

Durable finish, resistant to abrasion

 

Difficulty to rub out to adjust sheen or remove scuffs  (the converse of abrasion resistance)

Affordable cost options

Unused portion can cure in can giving thick or skinned over.

 

Poly has poor adhesion to other finishes or itself. Each coat must be lightly sanded so that the next coat adheres properly.  

 

Each coat is distinct from the one below it. This layer effect also makes it difficult to sand out blemishes in the finish: if you cut through one or more layers, a thin witness line will show along the boundary between the layers.

 

 

Formats of Varnish

Wiping Varnish

Wiping varnish is simply a varnish with a higher amount of carrier (thinner) added to make a less viscous product.   These are often sold under misleading names such as “Tung Oil Finish” (they are not or may not even contain tung  oil) or “Oil-Urethane Blend”

Gel Varnish

A Gel Varnish is a varnish that has had a thixotropic (thickening) agent added.   When at rest, it is a gel, and when energy is applied (e.g., rubbing) it reduces viscosity to a liquid, to return to gel when at rest again.

Brushing Varnish

If it doesn’t say the above, it’s probably a brushing varnish.  This is a varnish that has been thinned with a smaller amount of thinner than wiping varnish.

Long-oil Varnish

This is a varnish that in the recipe has a higher proportion of oil than “short-oil” varnishes, the more common type.  This makes a softer and more flexible finish.  Usually called “spar” varnish.  You may notice the oxymoron of products like Minwax Helmsman - a spar urethane.   Urethanes lack much UV resistance, and yet many people think this is a finish suitable for outdoor use.

Marine Varnish

This is a spar varnish to which UV absorbers/inhibiters has been added.  Usually pretty expensive ($50/qt) and available from marine supply houses such as Jamestown Distributors.  Common brands are Epifanes, Interlux Schooner, and Pettit.   It takes a number of coats (6-8) and regular maintenance and recoating to keep looking good in UV-rich environments.

Food Safe Varnishes / Salad Bowl Finish

All common varnishes are non-toxic once fully cured.   Another marketing gimmick.  One brand of salad bowl finish was exactly  the same as the company’s wiping varnish.  Just a different label on the can.

Varnish Stains

These are "all-in-one" finishes that are somewhere between a clear varnish and a paint.  Common example is Polyshades.  I generally consider this a type of "toner" - a finish with added pigment.  These are very difficult to apply without getting streaking or opaqueness.  So I don't recommend them.  I have a finishing book written in the early 1950s that talks about how terrible these are.   "The properties of a good stain are penetration and clarity.  It does not penetrate the surface, and the varnish diminishes its clarity  The result of using this stain is a muddy, streaky surface." (Gibbia, "Wood Finishing and Refinishing")  Plus ça change, plus c'est la même chose .

Water-based Polyurethane / Water-borne Varnish

These are completely different animals that are made and cure in very different ways.   We’ll look at those later.  More misleading labels.

 

How to apply varnish

There are three ways to apply any finish – cloth (wiping), brush, or spray.   In coming weeks, we’ll look at varnish’s options.

 

 

 

 

schnewj, lew, clhyer and 1 other like this

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Forgot to add next week I may be off-grid, so Tuesday's TGIF might be a bit late.

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I wish woodworker's everywhere would read info like this. It's gets really confusing since many refer to almost any finish as "poly", they'll ask a question and then the replies typically think they are referring to varnish. Only to find out they are using a waterborne..or something else. Good stuff! I think that first chart may be outdated, the Cabot 8000 series seems to have been scrapped (too bad, it was a great product....still have 2 quarts) and the SW Fast dry is actually a linseed oil/Alkyd varnish. I love it, but wish it wasn't so amber in tone.

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9 hours ago, kmealy said:

Gel Varnish

 

A Gel Varnish is a varnish that has had a thixotropic (thickening) agent added.   When at rest, it is a gel, and when energy is applied (e.g., rubbing) it reduces viscosity to a liquid, to return to gel when at rest again.

Keith, been using Gel Finishes lately, by General Finishes. Googling the product and seeing user reviews about it, some users report the finish expiring or over thickening if left too long in storage. But I have not seen a timeline on that expiration, and General Finishes does not mentions the over thickening due to storage, do you have any information on storage and use, when it may go bad, or why?

Thanks a ton Keith

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9 hours ago, kmealy said:

In coming weeks, we’ll look at varnish’s options.

Awesome!

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Posted (edited)

1 hour ago, Fred W. Hargis Jr said:

I wish woodworker's everywhere would read info like this. It's gets really confusing since many refer to almost any finish as "poly", they'll ask a question and then the replies typically think they are referring to varnish. Only to find out they are using a waterborne..or something else. Good stuff! I think that first chart may be outdated, the Cabot 8000 series seems to have been scrapped (too bad, it was a great product....still have 2 quarts) and the SW Fast dry is actually a linseed oil/Alkyd varnish. I love it, but wish it wasn't so amber in tone.

Yes, I would occasionally get a customer that would mention something about the "poly" on their factory furniture.  I usually politely tell than that poly is not a production finish and unless it was refinished by someone in their basement or garage, it's almost certainly not poly.

 

One of the nightmare jobs I ever had was a woman who bought a new dining table and then decided she was not happy with the sheen.  She decided to "slap on some poly."   By the time she called me, about 1/4 of the top had peeled off (adhesion mentioned)  The rest was so thick I sanded off with a ROS, and still took 3 rounds of stripper to get it off.

 

I was aware that my local hardware store wasn't stocking Cabot 8000 any more, but didn't know it was discontinued.  Used to be McClosky's, then Valspar bought the brand and relabeled to Cabot.  Now Valspar is part of Sherwin Williams.

Edited by kmealy (see edit history)

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4 hours ago, John Morris said:

Keith, been using Gel Finishes lately, by General Finishes. Googling the product and seeing user reviews about it, some users report the finish expiring or over thickening if left too long in storage. But I have not seen a timeline on that expiration, and General Finishes does not mentions the over thickening due to storage, do you have any information on storage and use, when it may go bad, or why?

Thanks a ton Keith

I have limited experience with gel varnish, but I'm not aware of anything that would effect shelf life.  If you can't find a "contact us" on GF's web site, I know and have the address of their tech guy.

John Morris likes this

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The difference between big box "spar varnish" and yacht supplier's / chandler's "marine varnish"

 

http://www.popularwoodworking.com/woodworking-blogs/flexner-on-finishing-blog/the-best-kind-of-clear-finish-to-use-outdoors-the-difference-between-varnish-and-spar-varnish

 

Spar urethane always seemed an oxymoron to me, why would you make something with low UV protection into a varnish that you lead people to believe is an outdoor finish?

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Good question, just another of the long list of marketing/misleading naming conventions the finish industry is prone to do. Flexner had mentioned the color test somewhere else, and indicated that even though Helmsman was labeled as having UV inhibitors, it did so poorly he concluded it had none. Yet the are posts where people praise how well it's held up (not many, but still!).

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Out here, the sun is our major finish destroyer. Sikkens ProLux holds up better than anything else we've tried. It does repel water but, that's not a real concern in our area. 

Can't understand why one would expect a finishing product, designed primarily for furniture, to withstand weather extremes.

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I went to a job a few years ago.   While there for a piece of furniture, the builder showed me the built in desk in the owner's (a doctor) office.   It was full of pen impressions and indentations after just  few weeks.   I asked what was on it and we looked around the cabinets and found half a can of Helmsman.  D'oh, idiot painters strike again.   Sorry, I can't help you with this.  Long oil varnishes like it are formulated to be softer because of the higher oil proportion.

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