oil...what kind?

[high.rise]

We've just had the doors dipped and stripped in our victorian house. Can anyone advise what oil we should use now they're paint free? I understand the oil will darken the doors down but I'd rather not use anything with too much pigment as we quite like the colour they are.

What's the difference between, Danish oil, rustic oil and linseed oil. These are the 3 we were considering (before your helpful advice!)

Many thanks.

 

[sidecar_jon]

The main differences are just slight tints in my opinion tho Exterior oils might have some slight difference they all seem to be linseeed oil and turps/thinners. working on boats ive seen a few "secret formulas" one friend openly used Mazola cooking oil to good effect.

 

[high.rise]

so do you think using boiled linseed oil is the best idea? Not entirely convinced on the old cooking oil inside the house thing though.....

 

[Nestor_Kelebay]

Highrise:

I will try to post a response to your question when I have more time tonight.

SOME, (not all), vegetable oils will transform into a solid when exposed to the oxygen in the air. (I will explain the process by which that happens tonight.) This process is caled "auto-oxidation", and every "oil based" coating (like Tung Oil, boiled linseed oil, true varnishes, alkyd paints, polyurethane "varnishes" and hardwood floor finishes, marine varnishes and mixtures of drying oils and synthetic resins like Danish oil and Swedish oil) ALL form films by this exact same auto-oxidation process.

Other vegetable oils will not transform into a solid when exposed to the oxygen in the air, and still others will form a sticky residue that just gets dirty fast. Such oils are called "non-drying" and semi-drying, respectively.

So far as I know, a cooking oil will not transform into a solid when exposed to air, so please don't paint cooking oil onto your doors.

Not all oils yellow to the same degree with age. Linseed oil is the worst for yellowing. If you want to avoid the yellow discoloration that occurs with time, then use Tung Oil instead. Tung Oil dries harder (but slower) and yellows less than Linseed oil.

Danish Oil and Swedish Oil contain a mixture of drying oils (like linseed oil, castor oil, Tung oil, or safflower oil) and synthetic resins (like alkyd resins or phenolic resins. (A "phenol" is a benzene ring with one of the hydrogens replaced with a hydroxyl (-OH) group.) The result is that the oil component penetrates into the wood (to darken it and bring out it's grain) and the synthetic resins penetrate slightly into the wood but remain on it's surface for the most part and auto-oxidize to form a protective film. Auto-oxidation also occurs between the synthetic resins on the surface and the drying oil that has penetrated into the wood, thereby ensuring excellent adhesion of the surface film to the wood.

A true varnish is simply a natural resin (which are basically dried tree sap, and in some cases petrified tree sap) dissolved in a drying oil like linseed oil or Tung oil.

An alkyd resin is best thought of as "clumps" of drying oil molecules (or parts thereof) (the fatty acid parts, actually) held together with synthetic "glue", most commonly something called "phthalic anhydride". These clumps are too big to penetrate into wood very well.

A "polyurethane" is what you get when you add something called toluene di-isocyanate to the pot when making alkyd resins. The isocyanate groups (-N=C=O) react with the hydroxyl (-OH) groups in both the phthalic anhydride and glycerine in the alkyd resin and form urethane groups (-(NH)-(C=O)-O-) within the clump that act very much like the roll cage in a race car, making the clump harder to squash. These clumps are too big to penetrate into wood very well either.

The result is that polyurethanes dry to harder films than alkyds.

And alkyds dry to harder films than varnishes.

And varnishes dry to harder films than the drying oils do.

So, to get the best protection, you want to go with the hardest film.

But, doors simply don't need the amount of protection that the wood on a hardwood floor needs, so you would also get good service from a drying oil like Tung Oil or a more protective finish like Danish or Swedish oil.

Alternatively, you could apply an oil finish (like Danish Oil) and then apply a polyurethane over top of it. This will give you a Danish Oil appearance, but a much harder and more protective film over the wood.

The only problem with that is that it's never a very good idea to put a hard film over a softer one because of the tendency for the harder surface film to chip off the softer one.

 

[sidecar_jon]

I wasn't suggesting you used Mazola, just that all sorts work quiet well. Mazola was used inside and mixed with turps on a sort of French polishing technique. True some oils don't dry as such Poppy oil paints went a success as they stayed wet for months for example. Id go for something like a light Danish Oil which shouldn't colour the wood too much just bring out the grain. Boating opinion is that wood oils sink into the surface while varnish are more like paint and stay on the surface (hence have to be stripped off regularly and re-varnished)..

 

[high.rise]

that's excellent, thanks for the concise reply. Really helpful stuff.

 

[oilman]

Tung oil would be the simplest for the result you are after. It will take a few weeks to put on the 10 to 20 coats that will give you a good looking door.

 

[Nestor_Kelebay]

High.Rise:

As promised, here is an explanation of how drying oils form films, and how alkyds are made from drying oils:

I had the part on drying oils saved from a post I did on another site, and just added the part about what alkyds are.

Here's what people need to know about auto-oxidation, which is the process by which all oil based coatings transform from a liquid into a solid film over the substrate:

First off, ALL plant oils and animal fats are "triglycerides", which simply means that the oil or fat molecule consists of three hydrocarbon chains connected to a glycerine molecule. We call those hydrocarbon chains "fatty acids" or "lipids".

In most vegetable oils, the hydrocarbon chains we're talking about will be
anywhere from 14 to 22 carbon atoms long.

Now, normally these hydrocarbon chains are said to be "saturated" because each carbon atom is bonded to the two carbon atoms on either side of it and TWO hydrogen atoms, like this:

-HCH-HCH-HCH-HCH-HCH-HCH-HCH-HCH-

...where the hydrogen atoms can be thought of as being located above and below the carbon atoms they are bonded to.

Now, with vegetable oils, it's common to have something called "unsaturated sites" in the fatty acids. An unsaturated site occurs where a double bond exists between adjacent carbon atoms, so that each of those two carbon atoms only has one hydrogen bonded to it, like this:

-HCH-HCH-HC=CH-HC=CH-HCH-HCH-HC=CH-HCH-HCH-

in the above hydrocarbon chain, there are three unsaturated sites.

A strange thing happens when there are two unsaturated sites in close proximity, and an oxygen molecule from the air comes between them. What happens is that the oxygen molecule breaks up into two oxygen atoms, and each atom bonds with one of the carbon atoms in each unsaturated site. The result is that the oxygen molecule ceases to exist, and instead you get two "C-O-C" crosslinks created between those two nearby unsaturated sites.

The spontaneous formation of crosslinks between unsaturated sites in close proximity is called "auto-oxidation". Obviously, the more unsaturated sites there are in the fatty acids of any particular kind of vegetable oil, the more of them are going to be in close proximity to one another, and the more times auto-oxidation will occur to connect unsaturated sites on the same and neighboring oil molecules. The more crosslinking that occurs, the lower the freedom of each oil molecule to move independantly of it's neighbors, and the more the oil becomes a solid as opposed to a liquid.

Not all vegetable oils have the same number of those unsaturated sites in their fatty acids. Most vegetable oils, like coconut oil and palm oil, have very few unsaturated sites in their fatty acids, and those oils will never transform into a solid no matter how long they are exposed to air. Other oils, like soy bean oil, have more unsaturated sites, but not enough to actually transform into a solid. We call such oils "semi-drying" oils because they transform into something, but not a solid. They transform into a useless mess.

But, some vegetable oils have a lot of those unsaturated sites in their fatty acids, and those are the oils useful in making coatings.

Go to this web site:

http://www.seatons-uk.co.uk/home.aspx?d=content&s=62&r=109&p=451

and click on the link "View typical fatty acid profiles".

The table you see gives typical percentages of different types of fatty acids in some drying oils (linseed, Tung, fish oil, Castor oil, and Safflower Oil) and some semi-drying oils (Rapeseed, soy bean and sunflower oil).

After the type of fatty acid, there will be something like C20:2
The "20" means there are 20 carbon atoms in the hydrocarbon chain.
The "2" means there are 2 unsaturated sites in that hydrocarbon chain.

Now, the more unsaturated sites there are in the oil, the more crosslinks will form inside it, and the harder that oil will dry when exposed to air.

From the chart, Tung oil molecules will typically be about 80 percent Eleosteric fatty acids, and those have 3 unsaturated sites each. The large number of unsaturated sites in Tung oil means that it dries to a harder film than any other oil, and a harder coating is generally a more protective coating.

Linseed oil has nearly as many unsaturated sites, but it contains between 50 and 60 percent linolenic fatty acids, and those are associated with the yellow discoloration in oil based coatings as they age. Because of the high linolenic fatty acid content in linseed oil, it yellows more than any other oil with age.

Fish oils are collected from sardines and herring.

There are other oils that dry to a solid through that auto-oxidation process, like walnut and poppyseed oil, but they are too expensive to be used for house paints. Such oils are used to make oil based paints for artists and can be purchased in art supply stores.


Soy bean oil (and rape seed and sunflower seed oils) are semi-drying. They are included on the list because these oils are both cheap and abundant. As a result, these are the oils that are "modified" and used to make alkyd and polyurethane resins for oil based paints and "varnishes".

What's an alkyd resin:

An alkyd resin is best thought of as a "clump" of fatty acids held together with synthetic glue.

Recall that a vegetable oil is a triglyceride which means that it consists of
three fatty acids all connected at one end to a glycerine molecule?

Well, to make an alkyd resin, they start by removing the fatty acids from that glycerine molecule in an inexpensive and abundant oil like soy bean oil.

Then, they use chemical means to increase the number of unsaturated sites in those soy bean oil fatty acids. So, instead of the soy bean oil containing 50 to 56 percent linoleic fatty acids with two unsaturated sites in an 18 carbon atom long chain, you might have 6 or 7 unsaturated sites in that linoleic fatty acid now. Think of this as "fatty acids on steroids".

Next, they mix those fatty acids on steroids with glycerine and something called phthalic anhydride, so that you have a "clump" of maybe 5 to 20 fatty acids on steroids held together with synthetic glue made up of phthalic anhydride and glycerine.

Now, an "acid" is anything that has a reactive hydrogen atom on it. Phthalic anhydride is called a di-basic acid because it has two reactive
hydrogen atoms on it.

And "alcohol" is anything with a hydroxyl group (-OH) bonded to a carbon atom, and a "polyol" has more than one such occurance. Glycerine, for example has three such hydroxyl groups bonded to carbon atoms.

A "condensation reaction" is where something with a reactive hydrogen atom reacts with something else with a hydroxyl (-OH) group, and the two somethings are joined together, and a H2O molecule floats away.

Technically, an alkyd is made by the condensation reaction of a dibasic acid and a polyol. You can use different kinds of dibasic acids and polyols to make alkyds, but the most commonly used dibasic acid is phthalic anhydride and the most commonly used polyol is glycerine.

Take a look at the blue image on this web page:

http://web.umr.edu/~wlf/CHEM381/chap33.html#glyphthal

(The first company to make alkyd resins was General Electric, and they called them "Glyphthal resins".)

In that blue image, the three carbon atoms lined up at the crossroads of
everything are meant to represent a glycerine molecule. The funny looking crab-like chemicals on either side are phthalic anhydride molecules, and the thing written as (CH2)n is meant to represent the fatty acid with the unsaturated sites in it.

Figure 1 in the following PDF file shows the structure of a typical alkyd resin.

The squiggly lines are meant to be the fatty acids: (note that this article is about using alkyd resins to make printing inks rather than paints, but an alkyd resin is an alkyd resin).

http://www.piranet.com/admin/_private/TechnicalArticles/00013q.pdf

The first chapter of this thesis (PhD thesis, I think) basically discusses what we've talked about so far:

http://alexandria.tue.nl/extra2/200510790.pdf

(It's a 183 page PDF file, so it takes a while to load!)

To make a polyurethane resin, you just add toluene di-isocyanates when making the alkyd resin.

It's late, I'm tired and there's a real good show on tonight about nutrinos
called "The Ghost Particle" that I want to watch.

I'll post more about alkyd and alkyd based polyurethane resins if anyone wants. (I suspect no one really wants.)

But, the thing to understand about alkyds and polyurethane resins is that they ALL form films by that same auto-oxidation reaction between unsaturated sites within the fatty acids used to make the alkyd and polyurethane resins. Consequently, you can paint a Tung oil over a polyurethane within 24 hours of painting the polyurethane on, and Oxygen crosslinks will form between the two coatings, bonding them chemically together.

 

[Scrit]

So basically, using an oil finish containing a high percentage of either polymerised tung oil or boiled linseed oil (BLO) or both will produce a thin, easily repaired finish with sufficient durability for use on doors. Avoid vegetable cooking oils because they just go sticky and don't dry - although that's ideal if you're frying fish. BLO will darken more than tung.

And one practical tip - BLO and polymerised tung oil both dry exothermically. That is to say they generate heat as they dry. So if you wipe them on with rags just remember to open the rags out and leave them falt and in the open to dry overnight before throwing them in the bin. Screw them up and throw them away and they sometimes spontaneously combust and your bin catches fire. Not that likely, but better safe than sorry.

Scrit

 

[high.rise]

thanks for all the excellent information. Just one further question about application of the oil, I intended to use lint rags, is that the best way to do it?

 

[Scrit]

Don't you mean "lint-free"? Well-0washed cotton or cotton poly shirts or T-shirts do the job

Scrit

 

[high.rise]

whoops yes. was supposed to say lint free...

 

[Nestor_Kelebay]

Scrit's 100% right about that auto-oxidation process being exothermic. It's this process that is the cause of the warning never to allow "OILY" rages to accumulate in a pile. (And, now you know that this same warning wouldn't apply to rags that are damp with white spirits or rags that are damp with a non-drying oil.)

I don't know about using rags. The vast majority of the painting I do with with emulsion paint using a roller. I know the chemistry of oil based paints, but I don't do much in the way of "wood finishing" like someone who stains and varnishes woodwork, or furniture.

An straight oil coating like Tung oil would probably suffice, but if it were me I'd want a harder more durable clear coat over the wood, so I'd probably opt for a Danish or Swedish oil which will have either natural or synthetic resins (or both) in it to leave a more durable film on the surface of the door. Alternatively, you could use an oil finish, and then just thin some polyurethane 50/50 with white spirits, and paint that over the oil finish. As long as you apply the second coat after the first coat is dry to the touch, but not yet fully cured, then you will get crosslinking between the two coats.

Also, in a previous post I stated that polymerized oil finishes and alkyd an polyurethane resins don't penetrate well into wood. I thought I should explain that a bit further:

The smaller the molecule is, the better it penetrates into wood. Typically the fatty acids are from 14 to 22 carbon atoms long, so an oil molecule with 3 fatty acids would be expected to have 45 and 69 carbon atoms in it (counting the three in the glycerine molecule that the fatty acids are connected to).

So, oil molecules are a size 45 to 69 when being fitted for a suit.

Alkyd resins have anywhere from 5 to 20 fatty acids in them, and they also have all that synthetic glue. I don't have any number for how big they are, but I'm guessing at somewhere between several hundread and several thousand atoms.

However, compare that to a typical emulsion paint resin that will have 50,000 atoms in each resin. Emulsion paint resins don't penetrate AT ALL because of that huge size.

So, alkyds, polyurethanes and polymerized oils won't penetrate into the wood nearly as well as an oil finish will, but there will be SOME penetration. Emulsion paints won't penetrate at all.

I just didn't want to give the impression that alkyds should be put into the same bag as emulsion paint resins when it comes to penetration into wood. Emulsion won't penetrate AT ALL, oils penetrate deeply, and alkyds and polymerized oil fall somewhere in between depending on size.