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frankiedog 12-04-2008 10:12 PM

I have just completed the installation of a wide plank floor. I am thinking about finishing them with boiled linseed oil. Any advice out there?

Nestor_Kelebay 12-04-2008 10:54 PM


The answer isn't just "No", it's "Of course not."

Boiled linseed oil is a drying oil that simply doesn't have the hardness to stand up well on a floor. Dirt is going to become embedded in that oil film under people's shoes making the traffic lanes look dull and dirty quickly, moving furniture across that floor is going to leave scratches and marks on the floor. Basically, the harder the film you put over your wood floor, the better it will resist scratching and dirt becoming embedded in it. That means that the floor will stay looking good longer and will loose it's appearance more slowly than a softer coating. Just paint one of your kitchen cupboard shelves with an interior latex paint and see how it stands up to normal wear and tear. It'll be a disaster area in a few months.

THE REASON that alkyd based polyurethane has become the clear coat of choice over wood is because it dries to a much harder and stronger film than the varnish it replaced. You need hardness and strength to stand up well on a working surface like a floor. Drying oils simply don't develop that kind of hardness (or strength) and putting a drying oil over your floor is just going to result in your floor losing it's good looks very quickly after you finish it. That's because normal "wear and tear" is going to wear it out so much faster than a harder and stronger coating.

Consider an alkyd based polyurethane, and if you don't know why that means "oil based polyurethane", ask and I'll explain it.

If you want to go for the jugular on this one, consider an isocyanate based polyurethane like Bona "Traffic", which will typically dry to a film hardness about three times that of an "oil based polyurethane".
Bona Traffic is called a "waterborne" polyurethane, but don't think it's a latex product. It really should be called a "catalyzed waterborne polyurethane". The isocyanate prepolymer is dispersed in water along with glycerine in one jug, and a small bottle of catalyst comes in a second bottle. You add the catalyst "hardener" to the jug of prepolymer and glycerine, shake and spread that on your floor, and the result is a coating that's about three times as hard and strong as conventional "oil based polyurethane".

That PDF file for Traffic mentions that it outperforms moisture cure finishes. Moisture cure finishes are much harder and stronger than DIY coatings like oil based polyurethane, right off the hop. They are normally considered "industrial coatings" rather than "finishes". Wasser is probably the biggest name in aliphatic moisture cure coatings, and they do have a clear one, but I don't know if it's recommended for floors:
Aliphatic moisture cure coatings are more expensive than aromatic moisture cure coatings, but they don't yellow with age the way aromatics do.

Here's a company that sells both "oil modified urethane" (or "oil based polyurethane") and a moisture cure hardwood floor finish. Click on the "Clear Wood Finishes" link and hear from the horses's own mouth that their moisture cure polyurethane provides the ultimate in scuff resistance of all the hardwood floor finishes they sell:

PS: "Oil modified urethane" is a bass ackwards name for "alkyd based polyurethane". Harko's name suggested you started with urethane and you changed it by incorporating oil into it somehow. In fact, to make this stuff, they start with an "oil based" alkyd resin and adding isocyanates to create urethane linkages inside that alkyd resin. So "oil modified urethane" is backwards. It should be urethane modified alkyd or alkyd based polyurethane. It's like calling a cheese burger a "ground beef modified cheese sandwich" rather than a "hamburger with cheese in it". The former is bass ackwards. You modifiy a hamburger to make a cheese burger. You don't make a cheese burger out of a cheese sandwich by adding ground beef. It's just not the same thing.

frankiedog 12-05-2008 08:20 AM


Thanks for the info it will be very helpful in my decision.

I do have another question. Would it be advisable to apply a poly type finish over a dried coat of linseed oil?


duane1982 12-05-2008 10:01 AM

I'm assuming that you already went ahead with the linseed oil?:eek:

Nestor_Kelebay 12-05-2008 11:26 AM


If the surface of the floor is dull, meaning all the linseed oil was absorbed into the wood and didn't leave a film over the wood, then I'd go ahead with the alkyd based polyurethane. But, that's kinda highly unlikely. If you put on a normal coat, and not a real thin coat, then I expect some of the linseed oil penetrated into the wood and most of it is sitting on top of the wood.

If there is a film of linseed oil over the wood, then that oil will form a soft layer under a hard layer, and that's likely to lead to chipping of the harder poly on top. The linseed oil simply isn't as strong and hard as the poly, and that means the poly won't be as well supported as it should be. Wall and ceiling paints don't really need to be well planned out, as long as they adhere. With floors, if you have someone wearing rubber soled shoes that decides to "turn on a dime" on that floor, you have to ensure that not only is the linseed oil is adhering to the surface of the wood well enough to resist that torsion, but that the linseed oil is strong enough to carry that twisting force without tearing itself. And, of course, the same holds true for the poly... it has to be sticking to the linseed oil well enough and strong enough itself to resist tearing off under those conditions.

If it wuz me, I'd probably buy a gallon of paint stripper and take the linseed oil off.

The active ingredient in paint strippers is methylene chloride, which is just a methane molecule with two of the hydrogens replaced with chlorine atoms. The stuff is highly volatile, and will evaporate completely without leaving a residue. Any residue on your floor would be either undissolved linseed oil or residue from the gelling agent used to gell the stuff, which you should be able to remove with paint thinner (or maybe just water, I dunno). I don't think it would affect any stain inside the wood.

frankiedog 12-05-2008 11:30 AM


No I havent put the linseed oil coating on yet but would like t for the amber color I believe it will produce.


Nestor_Kelebay 12-05-2008 12:01 PM


Does that room have a lot of natural sunlight (direct or indirect) coming in through windows?

Alkyd based polyurethane will give you the same yellowing with age that linseed oil will.

It's possible to do what you're thinking. But, I should explain to you what you need to do and why so that you'll understand the gameplan.

Linseed oil (and all drying oils, like Tung oil, oiticia oil, tall oil, walnut oil, poppyseed oil, etc.) modern alkyd resins and alkyd based polyurethanes all dry into solids through exactly the same process called "auto-oxidation". All of these oils and coatings contain something called "unsaturated sites" in the oil lipids (also called "fatty acids") that are inside these oils or resins. When two unsaturated sites are in close proximity, an oxygen molecule from the air will react to join those two unsaturated sites with a pair of C-O-C crosslinks. It's that crosslinking that occurs between oil molecules and between and within resins that converts the linseed oil, alkyd paint or polyurethane from a liquid into a solid.

But, only the drying oil molecules are small enough to penetrate deeply into wood for excellent adhesion to that wood.

And, because drying oils, alkyds and polyurethanes all cure by the same process because they all contain those unsaturated sites, if you put poly over oil, or alkyd over poly or oil over alkyd or alkyd over oil or any such combination, you will get crosslinks forming not just within each coat but also BETWEEN coats to create actual chemical bonding and not just adhesion of one coat to the underlying coat.

So, you can do what you want to do, but to avoid having a weak oil based film on top of the wood, you need to:

1. Apply the boiled linseed oil and give it a minute to penetrate into the wood

2. wipe the excess linseed oil off the wood (use dry rags or paper towels)

3. apply the polyurethane to the linseed oil wetted wood.

That will give you chemical bonding between the linseed oil in the wood and the polyurethane over top of the wood without having a hard layer over top of a soft layer in your floor finish.

By doing that, the oil penetrates into the wood, and ensures that the much larger polyurethane resins (which would penetrate very little into the wood) chemically bond with the oil molecules already inside the wood. That should ensure the poly sticks to the wood even better than you could accomplish just with normal adhesion of the poly to the wood.

However, if I were making a polyurethane hardwood floor finish, I would try to make the smallest polyurethane resins I could so that I'd get as much penetration into the wood as possible, so whether or not there's any actual advantage in doing the oil-poly trick is a question that's up in the air. Certainly, if the adhesion of the poly to the wood is sufficient to avoid problems, then having it adhere even better through chemical bonding is of NO practical advantage because no problems are being avoided.

In fact, there may be a disadvantage when it comes time to refinish the floor. You'd have to sand not only the poly off, but also the surface layer of wood that the oil penetrated into. That oil penetrated wood wouldn't be as porous and wouldn't take a stain the way it normally would. Polyurethane resins are much larger than oil molecules, and so they don't penetrate into the wood nearly as deeply, but that also means you don't have to sand as much wood off to get to virgin wood.

So, you can do what you're thinking, and there are advantages and disadvantages to it, but how important they are is something I just don't know. In my own humble opinion, you're probably best off sticking to the well beaten path by using a stain to give the wood the colour you want, and then just top coating with whatever hardwood floor finish you feel is best.

If you try the oil to polyurethane bonding trick, and there's a problem, the company making the poly will wash their hands of any warranty cuz you didn't follow their directions to the letter.

Note, that this chemical crosslinking between coats only occurs with "oil based" coatings, like drying oils, alkyds and alkyd based polyurethanes. You wouldn't have it between different chemistries of coatings like a drying oil and a water based poly or a catalyzed isocyanate based poly or a moisture cure poly. It's restricted to only those coatings that cure by auto-oxidation at unsaturated sites.

Also, maybe try just putting some alkyd based poly on your wood and see if that gives you the colour you want.

And, the yellowing with age that both oils, alkyds and alkyd based polyurethanes exhibit is bleached out by direct or indirect light from the Sun. So, if your floor yellows with age, and you have plenty of direct or indirect sunlight in the room, you will notice a difference in the colour of the coating on the floor when you move furniture that shades the floor (not like a table). That cuz it's dark under a piece of furniture (like a couch with a short skirt at the bottom that keeps the light out), and that yellowing only occurs under dim light. It is bleached out of the coating by direct or indirect sunlight. So, if you move the furniture so that the yellow colour is exposed to direct or indirect sunlight, within a week or two that yellowed colour that was under the furniture will be gone.

(You don't need to know this, but in the middle ages the rich had portraits of themselves painted so that their descendants would know what they looked like. The problem is that these expensive portraits would yellow with age in the dimly lit castles (often lit only by SMALL windows during the day (cuz glass was very expensive and you could put a cannon ball through a large window) and by candle light at night). So, it became standard practice to take those portraits outdoors in the spring time to expose them to sunlight to remove that yellowing that had occured since last spring. And that medieval practice became what we know today as "spring cleaning").

Linseed oil and polyurethane will all give the wood some yellowish colour, but the additional yellowing that comes with age and dim illumination can be eliminated from all oil based coating by exposure to direct or indirect sunlight.) Direct is where the sun is shining right on it, and indirect is where it's not in direct sunlight, but illuminated by reflected sunlight, such as off a wall.

frankiedog 12-05-2008 02:05 PM


You are a wealth of information. Thanks for the advice.
Just so you know the best part of your dialog was the origin of the term spring cleanig!!!


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