Canadian Woodworking

Finishing Touch: Thinning finishes

Author: Carl Duguay
Published: December January 2018

Let’s clear up some of the issues around thinning common wood finishes.


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Depending on the type of finish you use, how you apply the finish (brush, rag or spray) and the heat and humidity levels in your shop, you may find it helpful to thin the finish to make it easier to apply and keep it from drying out too quickly.

Oil finishes (principally tung and linseed), oil/varnish blends (any product that contains a petroleum distillate such as mineral spirits), and the new crop of hardwax oils typically don’t need any thinning. They’re meant to be applied in thin coats, by rag or brush. Water-based finishes are thinned with water or a commercial retarder like General Finishes Extender, which contains propylene glycol (a nontoxic compound used in the food processing industry). All these finishes have the advantage of having low VOC (volatile organic compound) levels or none at all.

However, for the more ‘traditional’ finishes – oil-based varnish and polyurethane, lacquer and shellac – the thinners are both toxic and highly flammable. They affect us through the fumes we breathe in, and from contact with our skin. It’s not just the short-term effects that we ought to be concerned with but also the cumulative, long-term impacts. This is especially important for those of us who work in confined spaces where vapours can’t easily escape.

Regardless of what you may have heard about these compounds, it makes sense to always wear an appropriate respirator, eye protection and gloves, even when using small amounts. Additionally, all these products should be considered flammable. Clean your brushes and spray equipment after use, and spread any rags in a single layer so that heat dissipates while the rags dry, after which you can safely discard them. And remember, don’t flush chemicals down the drain (or dispose them in your back yard). Take them to your local recycle centre.

While there are dozens of chemical compounds on the market, we’ve listed the ones that woodworkers are most likely to use for thinning finishes (or dissolving, in the case of shellac flakes). There are two major manufacturers of these products – Recochem is the major producer in Canada, and their solvents and thinners are available through most building supply stores. They also manufacture other brands of solvents and thinners under license. Less commonly available are products from the US-based Klean-Strip.

Mineral Spirits
Though products like paint thinner, Varsol and Naptha are similar to mineral spirits, they are not as pure. For best results use mineral spirits whenever you can. (Photo by Recochem)

Thinning Shellac
 Great for thinning any shellac product, this product is also compatible with some lacquers. (Photo by Lee Valley Tools)

The Well-Protected Woodworker

We recommend that you invest in a half- or full-mask respirator (such as the 3M 7500) that is equipped with an organic vapour cartridge (such as the 3M 6001). Protect your eyes with chemical splash goggles or safety glasses, and your hands with nitrile or neoprene gloves. The latter products are available from Lee Valley.

Thinning oil-based varnish and polyurethane

The thinner to use for oil-based varnish and polyurethane is mineral spirits – 3 or 4 parts varnish to 1 part mineral spirits. The ratio isn’t crucial. If you prefer to apply these finishes with a rag, thin them with about 50% mineral spirits, essentially creating your own ‘wiping varnish’. Of course, the more you dilute the finish, the more coats you’ll need to apply to build it up, as you’re putting less finish on the wood during each application. Lay on an extra couple of coats if you’re using a thinned brush-on finish, and upwards of a dozen or more coats if you’re using a wiping varnish. Use mineral spirits to clean your brushes and as a degreaser for cleaning the oil that typically covers new machinery. It’s also available in a low-odour formula – the aromatic solvents are removed, which gives it a somewhat slower evaporation rate.

Because mineral spirits is so commonly used there is a risk of becoming complacent (especially when using the low-odour formula). While it is somewhat safer than other chemicals, the fumes are still toxic, it can cause skin irritations, repeated exposure can cause damage to your nervous system, and it’s flammable.

While mineral spirits is a bit more expensive, the only reason to use any of these alternative thinners would be if you can’t get access to mineral spirits.

Paint thinner is essentially the same product as mineral spirits, but with a lower level of purity.

Varsol, a trademark name of Imperial Oil, is mineral spirits.

Naphtha (Camping Fuel) is more volatile than mineral spirits so it shortens the drying time. It’s also more flammable.

Turpentine was the traditional thinner for varnish, but it dries slowly, has a pungent odour and is more expensive.

Dissolving and thinning shellac

Shellac flakes are dissolved with alcohol. You also thin liquid shellac with alcohol. The exact ratio of alcohol to flakes (referred to as the pound-cut) isn’t critical. The preferred alcohol to use is ethyl alcohol (ethanol), the same stuff booze is made from. It’s widely available in the USA as Denatured Alcohol (additives make it unfit to drink), but difficult to obtain in Canada. However, some Aceaffiliated hardware stores do carry the Klean-Strip brand. Another option is Recochem’s BioFlame fuel, which is 93% ethanol/propanol.

More widely available is isopropyl, which is widely available as rubbing alcohol in pharmacies and grocery stores. Choose the 99% solution, which has less water than the 70% solution. I find that isopropyl takes noticeably longer to dissolve shellac than ethanol.

Methanol (methyl hydrate, wood alcohol) is best avoided, as it’s highly toxic and more flammable than ethanol or isopropyl.

Thinning lacquer

Lacquer thinner is the thinner for any type of lacquer, whether for a nitrocellulose brushing lacquer, or a nitrocellulose, CAB-acrylic, or catalyzed spray lacquer. You also use it for cleaning your brushes and spray equipment. It’s a more complex product, as it’s made up of a variety of solvents and alcohols, such as acetone, toulene, xylene, ethanol and methanol. For spray lacquer it’s best to use the specific lacquer thinner recommend by the manufacturer of the lacquer you’re using, as those will have been tested for compatibility. For a brushing lacquer you can use a lacquer thinner available at your local paint outlet.

What’s in a Name

As the name implies, a ‘thinner’ is used to thin (reduce the viscosity) of a finish. A ‘solvent,’ on the other hand, breaks down (dissolves) a substance. However, some thinners can also act as solvents, and some solvents are used for thinning. For example, mineral spirits is a thinner for varnish, but it will also dissolve wax. Denatured alcohol is a solvent for shellac flakes, but is also a thinner for shellac and lacquer.

Carl Duguay - [email protected]

Carl is a Victoria-based furniture maker and the web editor at Canadian Woodworking & Home Improvement.


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  2. I enjoyed your article on finishing. When I apply various finishes I do try to to have a window open and wear a mask or respirator. Would you be able to do an article on the many respirators available and the advantages of each? Thank you.

    1. Hi Rick: I just checked our database and was surprised to see that it’s a topic we’ve done very little on. We’ll aim to correct that this year. Stay tuned!

  3. Nice article!! I would add a safety precaution that is often overlooked. When using any thinner in a secondary container to mix , it is very important to use a static free product such as purpose made mixing containers sold at woodworking and paint stores. DO NOT be tempted to use an old yogurt container. I used to own several Woodcraft Stores in the US, and had a customer who had a flash fire when mixing shellac. fortunately he had a fire extinguisher and other than making a hell of a mess, there was little harm done. The fire department investigated and found the culprit. He used an old Folgers coffee container that had a static discharge that ignited the alcohol fumes. Lesson learned. Use metal or plastic purpose made products for mixing flammables. They will not discharge static!

  4. The availability (or lack thereof) of ethyl alcohol is often raised in Canada. In my experience, 95% denatured ethyl alcohol is just as easily available in drugstores (as ‘rubbing alcohol compound’ or something like that) as isopropyl – which is usually available in 70 or 99% concentrations. Most drugstores have all 3 varieties of ‘rubbing alcohols’ on hand. Just read the fine print on the bottle – the ethyl content is usually listed in the fine print rather than as part of the product title as the isopropyl tends to be.

  5. Hello Mr. Duguay. I have read your article on thinning poly with mineral spirits and as I’ve used poly before your suggestions seem like a good thing to do and I hadn’t heard of thinning before. If you’d be so kind (and I apologize if I just missed this), I have one question: You suggest thinning with 1 part spirits to 3-4 parts poly. However, I will be working on a vertical surface and I’m wondering if that should change the ratio at all. My sincere thanks for you time in reading this, and responding if you have time. Dave

    1. It depends in part on how large a surface. The big problem, as you know, will be dripping. The easiest would be to use a wipe on poly. If you find the 1:3 ratio hard to work with add a bit more solvent. Apply thin coats with a rag or pad, working from the top down. I find this works well for small projects. For large outdoor projects I always spray the finish (which might not be an option if you don’t have a spray system). Good luck Dave.

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