One-Part Polyurethane Varnish

Jim Hingst explains the differences in varnishes, and the procedures required for  trouble-free application of one-part polyurethane.

For finishing  an indoor project, polyurethane is a good choice, especially if you want durability. The coating is tough, easy to apply and it dries much faster than other varnishes. Be aware, though,  that oil-based polyurethane is not crystal clear. In fact, it does have a yellowish appearance. Over time it will yellow even more.

Personally I don’t find the yellowish or amber-colored hue objectionable. Many people like the look, because it warms up the color of the wood.  Another knock on this product is that if your build up a thick coating, the appearance is somewhat plastic. I can’t disagree with that opinion. It’s no surprise that it looks that way. The fact is the coating is plastic – it’s polyurethane.

  

If you desire a clearer, non-yellowish finish, then you can always use water-base polyurethane. Cleanup is easily accomplished with soap and water. The water-based product is also a little more environmentally-friendly, because it has lower VOCs. The disadvantage is that  the durability of this product is not equal to that of an oil-based product. What’s more, you will need to apply at least three coats of the water-based polyurethane.

How Polyurethane Differs from Other Varnishes. 

All oil-based varnish is a combination of oil and a resin.  The differences between a traditional varnishes and  polyurethanes are based on the types of resin used in the products. The original ‘natural’ varnishes were a combination of oil, such as linseed oil, and a natural resin, such as the fossilized sap of a pine tree.

Today, nearly all of the resins used to make the different types of varnish are synthetic.  These resin types include phenolic, alkyd and polyurethane.

Least common of the resins used today is phenolic.  The characteristic of this resin is its toughness and flexibility. This is the resin  which is combined with tung oil to make spar varnishes.  

Of these, the most common is a form of polyester, called an alkyd resin. Alkyd resins are is concocted from alcohol and acid and then cooked with oil to make varnish.  These resins are also used in other coatings, such as paint and lacquer.

Of the three types of synthetic resins, the toughest is polyurethane. The type of polyurethane that is commonly used in woodworking is a one-part finish. It is very different from the two-part catalyzed finishes that are used in the automotive industry. This one-part  finish is actually is a combination of alkyd and polyurethane resins. Rightfully, it should be referred to as an uralkyd.  

Supplies You Will Need. 

Before you get started, collect all of the supplies that you will need, including:

• Oil-based polyurethane
• 220-grit sandpaper
• Grey, Scotchbrite® pad
• Mineral Spirits
• Denatured Alcohol
• Clean, lint-free rags
• Tack cloth
• Natural bristle brush
• Latex gloves

Stirred, not Shaken. 

James Bond liked his martinis shaken, not stirred. When it comes to polyurethane varnish, stir, do not shake the can.  Stirring is not an optional activity; it is a must do.  That way you evenly mix all of the ingredients. If you are using a satin or semi-gloss product, it contains flattening agents, which will settle to the bottom of the can. 

The reason that you should not shake the can, is that you will create bubbles. When the bubbles pop, they usually can leave tiny craters on the surface of your finish.

Varnishing Brushes.  

Foam brushes are fine for staining.  They are not, however, recommended for varnishing.  The reason is that the textured surface of the foam brush contains air pockets which can generate bubbles. As I just mentioned, bubbles can burst and leave pockmarks on the surface of your finish.

For applying varnishes, the rule of thumb is to use a natural bristle brush to apply an oil-based product, whereas you should use a synthetic bristle brush for water-based varnishes.

Applying Polyurethane Varnish. 

Applying several thin coats of polyurethane varnish is preferable to applying thick coatings.  Here’s why. Thick coatings take too long to dry. Laying it on too thick can also result in drips, sags or wrinkles in the finish.

Applying several thin coats of polyurethane is preferable to applying thick coatings.

As you are applying the polyurethane, the prevailing wisdom is to brush with the grain. In brushing use long, slow strokes. Short, fast strokes will generate an excess of bubbles. 

No matter how careful you are, accept the fact that you will have some bubbles in the finish. Many of these bubbles will pop on their own.  You can also eliminate many of these using a “tipping off” stroke of the brush, in which you are very lightly brushing with just the tip of the brush.

Apply at least two coats of a polyurethane varnish, scuffing the surface between coats for good intercoat adhesion.

Scotch-Brite scuffing between coats.

When you are using an oil-based polyurethane, you will need to apply at least two coatings. Always lightly sand between coats of polyurethane, using 220-grit sandpaper. You can also scuff up the surface with a grey Scotch-Brite® pad between coats. What I like about the Scotch-Brite® pads is that in using them, you are just scratching the surface. With sandpaper, you are abrading some of the surface away rather than just putting  scratches in it. So be careful when sanding. You don’t want to bury the hatchet and sand right through the coating. While some of the bonding of one coat to the other is mechanical in nature, most of the intercoat adhesion involves chemical bonding.

If you are working on an irregular surface, such as a carving, a  Scotch-Brite® pad also allows you to get into tight spots that you cannot reach with sandpaper. On the other hand, if you are working on a smooth, flat surface, sandpaper will level any imperfections in the coating. Also, if you did happen to trap air bubbles in the coating, and end up with a rough, pitted surface, sanding with smooth the surface.

After using sandpaper or the Scotch-Brite® pad, always wipe down the surface with a cloth dampened with either denatured alcohol or mineral spirits.  While I have also used a tack cloth, many professional woodworkers advise against using it. Here’s why. If you rub too hard, you could deposit a residue from the cloth onto the surface.  This residue could interfere with intercoat adhesion. Do not sand the final coat of polyurethane.

Keep in mind that polyurethane cures to a very hard finish. Once this finish hardens, nothing really bonds well to it, including another coat of polyurethane. For this reason, you should allow 24 hours before applying an additional coat. By the same token, do not wait more than 48 hours. 

Thinning.  

To thin or not to thin, that is the question. If you are using Minwax’s fast-drying polyurethane, the manufacturer does not recommend this practice.  The reason is that as you add more thinner, the less  polyurethane remains on the surface after the solvent has evaporated.

For a smoother coating of polyurethane, I confess that I have thinned it with a little minerals spirits. Typically the addition of 5% to 10% of mineral spirits is all you need.  While some people condemn this practice, I don’t have a problem with it. Sure the thickness of the coating is slightly thinner. So just apply more coatings.

On the final coating, you can add up to 20% mineral spirits. With this amount of thinner, the polyurethane really flows out onto the surface better. On a flat surface the coating readily levels out. Any brush marks disappear. Plus, with the extra open time any bubbles that you generate in brushing, have more time to pop.  Not only is the application of the polyurethane smoother, but the thinner also promotes faster drying.  

After each application of polyurethane, always inspect your work for drips and places where the varnish may have pooled. If you miss a drip and don’t discover it until later, you can always do some repair work with a razor scraper. After removing the hardened drip, sand the area until the surface is smooth and then apply more varnish.

Clean Up. 

If you get varnish on your hands, resist the temptation to wash up with solvents, especially the strong ones such as lacquer thinner. You skin can absorb the solvents into your blood stream. Repeated exposure to these chemicals can result in dermatitis. Instead apply baby oil to the sticky residue. The oil will aid removal of the polyurethane from your skin.

After staining the green man with gel stain, the carving was coated with polyurethane and then waxed. 

Waxing the Finish. 

After you finish applying the polyurethane, you can wax the surface. I will cover paste waxes in an upcoming article. 

The carved cross was stained with a gel stain, varnished and then waxed with a clear wax and a brown mahogany wax. 

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.

Slides of Jim's Artwork

© 2015 Jim Hingst