# Oil Paint Yellowing



## sirwired (Jun 22, 2007)

Is a waterbourne enamel an option? You get oil-look, oil-like durability, and no yellowing. I don't think the gloss level makes a difference with the oil paints. And yes, it has to do with the light levels. More light = less yellow.

SirWired


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## Nestor_Kelebay (Jun 17, 2008)

Munruh:

Actually, the process of yellowing in oil based paints depends on a number of factors, the most important ones being the amount of linolenic acid in the oil and the second being the amount of natural sunshine the oil is exposed to.

All animal fats and vegetable oils are "tryglycerides". Even animal oils, like whale oil, are tryglycerides, meaning they all consist of three fatty acid chains (which may all be different kinds of fatty acids) all connected to a central glycerine molecule. Take a look at this web page:

http://www.seatons-uk.co.uk/home.aspx?d=content&s=62&r=109&p=450

The diagram below the first table on the page shows the basic structure of a triglyceride, where R1, R2 and R3 can be any of the fatty acids in the table above it. The fatty acids in that table are the ones most commonly found in vegetable oils. So, for example, a single triglyceride of a vegetable oil could contain three different kinds of fatty acids, or all three could be the same fatty acid, or two of the fatty acids could be the same and one different. Where it says C18:3, or C16:1, it means that the fatty acid is 18 or 16 carbon atoms long, and has three or one double bond between carbon atoms, like this (-HC=CH-). It's those double bonds where oxygen from the air reacts to crosslink the oil molecules together, so the more double bonds there are in the fatty acids, the faster the oil will dry, and the harder film it will dry to.

Now, go to this web page:
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 resulting table shows the relative amounts of each different kind of fatty acid found in vegetable oils.

It is seen that linseed oil (pressed from flax seed) is typically more than half linolenic fatty acids, and it is these linolenic fatty acids that yellow at the highest rate. Tung oil, by contrast is mostly eleostearic fatty acids, and these yellow much slower than linolenic fatty acids, so Tung oil yellows at a slower rate than linseed oil does. So, the amount of yellowing depends on the kind of oil used in the oil based coating.

Modern alkyd paints are made out of either soy bean oil or corn oil that has been chemically modified to have more of those (-HC=CH-) unsaturated sites in it. However, soy bean oil is mostly linoleic and oleic fatty acids, both of which also yellow less then the linolenic fatty acids. So alkyds yellow at a slower rate than linseed oil based paints.

It's really how quickly the fatty acids in the oil yellow that determines how quickly the paint yellows, and different fatty acids yellow at different rates with the linolenic fatty acids abundant in linseed oi being the worst.

The good thing about yellowing in oil based paints is that it's completely reversible.

Take a look at Figure 1 on this web page:
http://aic.stanford.edu/jaic/articles/jaic24-02-002.html

Two different drying oil based paints and one alkyd paint were allowed to yellow in low light conditions, and were subsequently exposed to indirect light from the Sun. It's the high energy rays in sunlight that remove the yellowing in oil based paints, and artificial lighting simply doesn't have the energy necessary to remove yellowing in oil based paints. From Figure 1, you can see that even after repeated yellowings, each different kind of paint could be returned to it's original unyellowed condition by exposure to indirect sunlight.

Presumably, each different kind of oil based paint would have been returned to it's unyellowed condition even faster if exposed to direct sunlight, but the JAIC (Journal of the American Institute for Conservation) is a journal for museum curators who want to remove the yellowing from paintings that have been kept in storage without risking causing the paint to fade due to the UV light from the Sun. The point of this paper is that it's possible to remove the yellowing of old paintings kept in storage by exposing them to indirect sunlight which shouldn't cause nearly as much fading as direct sunlight would.

However, the point to be made here is that it's the amount of NATURAL light in the room that will determine the amount of yellowing of the oil based paints in that room. In well lit area, such as on window sills or anywhere in a well naturally lit room, the amount of yellowing in an oil based paint will be nil. It will be bleached out by the direct and indirect sunlight faster than it yellows. It's only when you use oil based paints in rooms with little natural sunlight (such as bathrooms) or under conditions where the sunlight can't get to the paint (such as inside closets or cabinets) that yellowing is the worst.

So, the gloss of the paint plays no role in the yellowing. What matters is the kind of fatty acids in the coating and the amount of natural sunlight in the room. Also, if you are planning to paint the interiors of cupboards (or cabinets), consider installing glass doors on them so that any indirect natural sunlight in the room can remove the yellowing of any oil based paint you use inside those cupboards or cabinets.

In my case, I use the same oil based paint on all my window sills, in my kitchen cupboards and in my closets. There is NO perceptible yellowing at all on the window sills and when I repaint a window sill you can't tell the new paint from the old by colour alone. However, when I repaint an area in my cupboards or closets, it's blindingly obvious where the new paint is because it's a completely different colour than the old paint. AFter a few months, both paints are the same colour and you can't tell where I repainted.


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## slickshift (Aug 25, 2005)

munruh said:


> Does anyone know if using a satin gloss would produce better results [less yellowing] than a high gloss


Not significantly, not within the same line-up (apples to apples)

Your best bet for non-yellowing would be to switch to a quality waterborne enamel


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