The Answer to ‘How Many Coats Of Varnish Should I Apply’ is simple – As many as it takes to use 1/2 Litre of good varnish per square metre of timber, that’s a pint per 20 square feet in old money.
That’s just one of the 3 things that you need to get a long lasting varnish finish.
The 3 Steps to Long Lasting Varnish
The instructions below will give a varnished wood finish that can last up to 20 years exterior use with maintenance anywhere in the world.
1 – Prime the wood properly
Most varnish manufacturers suggest thinning the first coats of varnish to prime the timber. Prime the wood with Clear Penetrating Epoxy Sealer to get a varnish finish that really stays stuck to the wood and can last up to 20 years with maintenance.
2 – Answer ‘How Many Coats of Varnish’ this way
Calculate the total area of your timber . Buy a good quality varnish, and buy 1/2 a litre per sq metre of timber. Keep adding coats until you have used all of the varnish. This will give you the correct dry film thickness of varnish to offer excellent protection against UV light. The technical description of why dry film thickness is the important measure follows later in this article.
3 – Maintain your varnish finish
As your varnish is exposed to UV light and the elements it will degrade. Read further in this article to see why. Once the varnish loses it’s gloss finish – it’s time to recoat. Again, it’s not how many coats of varnish should I apply to maintain. Simply use half the quantity you originally used, 1/4 litre per sq metre of varnish. We cannot tell you how often this will be as it depends on many variables:
- The quality of your varnish
- Your latitude
- The amount of exposure to the sun your timber get’s (a boat or table get’s direct midday sun, a door get’s low angle sun, if situated under a porch it get’s even less exposure)
So – if you want to see how this is done please read our how to varnish properly article.
The varnished piece below was done in 2019 – and still looks absolutely perfect.
If you want the full technical details of why its dry film thickness that matters, then please read on.
Chemistry: Varnish life – Why do some people get better results?
So how many coats of varnish should I apply?
There’s a lot of folklore about how many coats of varnish to apply. Six coats, eight coats, twelve coats…everyone’s got an opinion.
They might all be right, they could just as easily all be wrong.
It’s pretty funny, when you realize there’s no definition of the film thickness of a coat, and yet all the experts agree that dry film thickness determines life. How many coats of varnish is the wrong question, it’s how much varnish should I apply that you should ask yourself.
The joke is on you, varnishing something, whether your front door or the wood on your boat. Even if you put on ten coats, how do you know you have enough of a film thickness?
So why woudl you listen to me?
Well, I’m Steve Smith, a paint chemist and proprietor of Smith & Co. I’ve been in this business since 1972, and this is one of the things I learned. You can learn about my products at the main company website Smith & Co. and the other websites linked to it. You can look at those later. Right now I’m going to tell you the simplest possible way to know how much varnish to put on something, and when to recoat. I’ll tell you about recoating first, because you need to have some additional data to fully understand the answer.
Varnish on a boat, with all-day exposure to the sun loses its gloss after a year, give or take. Better varnishes hold up longer, close to two years. The lesser quality ones may lose their gloss enough to be really noticeable in only three to six months. For a front door or entryway on a house, getting full sun for maybe half-a-day and then only when the sun is low in the sky and there’s more atmospheric filtering, you might triple those numbers, as doors don’t get near as much ultraviolet as does the unprotected wood on a boat.
When the gloss has significantly gone, that’s when it’s time to recoat. The later applications are called “maintenance coats”. You can apply about half as much on a recoat as you did the first time around, and the varnish-job will stay stuck to the wood, provided we did the priming correctly with Clear Penetrating Epoxy Sealer™, also known as CPES™, and that we put on enough in the first place. Do that right and you will not need to sand between coats of varnish. You can just search on the Internet for CPES. This essay is about varnishing.
It’s dry film thickness not how many coats of varnish!
Is there a reason why dry film thickness determines life? Yes, indeed there is. There is a legitimate criterion for film thickness, for those clear finishes that have ultraviolet absorbers, and if they don’t have any ultraviolet absorbers you should not use them on wood, because they will fail in as little as a few months.
One needs a sufficient film thickness of a film containing ultraviolet absorber chemicals so that the absorbers can attenuate the ultraviolet to little-enough that it does not degrade the wood within the life expectancy of the finish, based on other failure mechanisms that would require stripping. Loss-of-gloss is not such a failure mechanism, since it can be fixed with a recoating. Mechanical abuse is one of the ones that comes to mind, the other being progressive loss of flexibility due to the ultraviolet from the sun causing breakage of the polymer chains, creation of free radicals, and additional cross-linking leading to increased film stiffness, loss of elongation, and eventually cracking and peeling from the wood. That, incidentally, is the reason that a peeling varnish film curls outwards as it peels. The outer surface molecules got shorter than the ones below due to that additional cross-linking. Betcha you always wondered about that……
The result of the ultraviolet degrading the wood is that the fibers lose their color, turn grey, and separate from each other as the natural wood-resin glue that holds them together gets broken down and oxidized away. The process is somewhat like combustion, only slower. A couple of years ago I put a piece of varnished teak under ultraviolet lamps, expecting to eventually get it to flake and peel (I wanted a picture of that for marketing purposes). It lost its gloss promptly, but then the color stayed constant and, after a few weeks, I found the varnish was all gone and only bare wood remained. What I realized from that experiment is that the recombination of free radicals within the polymer film does not take place in the time frame of hours or days, but rather within the time frame of weeks to months. The dominant chemical process I had inadvertently activated was oxidation of the surface molecules and free radicals which the high-intensity ultraviolet was creating.
Under low-intensity ultraviolet as in sunlight we see the film age differently, and this is particularly noticeable when the film is too thin. Have you ever noticed a clear finish, of any sort, fail sooner in some locations but not others, particularly near edges or sharp corners? The film is thinner there, and so more ultraviolet comes through the film there. What is happening is that a percentage of the ultraviolet is absorbed per unit of film thickness, depending on the level of ultraviolet absorbers in the film.
And thereby hangs a tale.
Oil varnishes cure by oxygen connecting the double bonds of the oil. Those polyunsaturated oils that are good for your cholesterol have lots of those carbon-carbon double bonds. That is why they are said to be unsaturated, and poly- means many….meaning they have many of those double bonds between carbon atoms.
Not all of those double bonds react with each other as the oil dries and cures. Eventually the remaining double bonds are too hindered in movement and too far away from each other to be able to find each other. They remain in the cured film, and are capable of absorbing ultraviolet radiation. When that happens enough times (one ultraviolet photon absorbed by one double bond, it vibrates for a while, dissipating the energy as heat), the thing breaks. That creates a yellowing in the film, as well as free radicals that cause embrittlement. That is probably the biggest reason why varnishes yellow with age.
Ultraviolet absorbers can also be added by the manufacturer, and these are the expensive synthetic chemicals that absorb ultraviolet energy very efficiently (although these wear out, too). They do not absorb the energy all at once; for any film thickness and concentration of ultraviolet absorbers there is a percentage attenuation per unit thickness. When the ultraviolet is reduced at the surface of the wood to a few percent of the incident level, other failure mechanisms dominate. Where the ultraviolet at the surface of the wood is maybe half or a quarter of the incident level, then the wood will take twice or four times as long to bleach, lose its color and grey, as if it had no protection at all.
Eventually the ultraviolet absorbers in the film (from whatever source) wear out, and then the wood bleaches faster, grays faster, and you notice a lighter or even grayed patch under that clear finish in spots.
Well, that’s what is happening. Film thickness does matter…..up to a point. Beyond that, other failure mechanisms dominate.
Not just UV light – Antioxidants too
Oil varnishes, by the way, cure by metallic dryers catalyzing the reaction of the oxygen in the air with those double bonds. That is called oxidation. The antioxidants in some 2-part polyurethane finishes that trap the free radicals and can mostly prevent (until they are depleted) loss of film flexibility, they are ANTIOXIDANTS, which means they block the curing reaction. That is why varnishes have that failure mechanism, and why the two-component polyurethanes are superior in that respect…..they can accept both antioxidants and ultraviolet absorbers, which varnishes cannot.
Eventually, sometimes, the film thickness of an old varnish will become too hard to stretch with the wood, and then the old film cracks but below the surface, nearer the wood (assuming one recoats occasionally). This depends a LOT on the individual circumstances, the varnish, and on the wood. Now, the last thing you need to know is how much varnish to put on the wood to start with, and how to know that.
The amount of ultraviolet absorber in the varnish needs to be a certain amount, to protect the primer and the wood from the ultraviolet; that’s understood now. From my experience and that of my many customers since 1972 with many varnishes, the best one in the world is made in Holland, and is called Epifanes Gloss. While my company does sell it, this particular varnish is widely available. I have done aging tests myself, and I consider it the Quality Standard, among all the commercially available varnishes.
Here’s how much of it to apply the first time: a ten mil dry film thickness. Here’s how you get that: Use a quart for every twenty square feet. That assumes you are not sanding off what you built up, by sanding between coats. I don’t care how you thin it or how many coats it takes, for there’s no definition of the film thickness of a coat, anyway. Just thin the varnish (Turpentine or mineral spirits is the usual thinner) so it applies well, and don’t spread a quart over more than twenty square feet the first time. When you recoat some years later, a quart for every forty square feet will give another five mils dry film thickness, and that replaces the ultraviolet absorber faster than it burns out, and so the wood stays protected and stays looking beautiful, and that’s what you wanted in the first place, right?
In metric please?
Ok – in modern money – that’s 1/2 litre per square metre to varnish, 1/4 litre per square metre when maintaining.