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I've been looking into making an oilcloth tarp. All the sources I have read so far seem to consider linseed oil the only oil to use, and I'm wondering why that is.

So, what, if anything, is it about linseed oil that makes it a good oil for oilcloth?

  • I'm not convinced that linseed oil is a good choice for a tarp: it will not stay elastic and thus will be prone to crack where the tarp is folded. (There's an intermediate stage where it is elastic but also sticky and smelly when not yet fully oxidized). Stand oil en.wikipedia.org/wiki/Linseed_oil#Stand_oil is a linseed oil product that may give a more elastic coating. – cbeleites Jun 18 at 8:21
  • Also, the German wiki page on oilskin says that linseed oil went out of use in favor of natural rubber in the 19th century (i.e. comparably fast). English Wiki page says that the Le Roy process used a mixture of linseed oil and wax. As the wax doesn't participate in the forming of the epoxy residue, it will stay more mobile. Thinking along this line, it may be possible to arrive at a mixture of linseed oil and, say, sunflower oil (not sufficiently unsaturated to cure) that will be less prone to cracking and still sufficiently viscous not to run off the cloth. It may stay sticky, though. – cbeleites Jun 18 at 8:49
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    Also, note that cloth soaked in linseed oil has a bit of a habit of spontaneously bursting into flames. – David Richerby Jun 18 at 11:19
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    @DavidRicherby Notd. The recommended drying times before folding the cloth up seem to be measured in weeks, – Haem Jun 18 at 11:57
  • Linseed oil does take weeks to cure, which is why the "boiled" variety is more common as a wood finish. Boiled now referring exclusively to linseed oil with chemical driers added. – Matthew Gauthier Jun 18 at 12:19
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Linseed oil is used because it's a drying oil,

A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink (and hence, polymerize) by the action of oxygen (not through the evaporation of water or other solvents).

Source

It also looks like the alternatives are not considered to work as well as linseed.

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    (Too lazy to sign up for crafs.sx) The alternatives they discuss are mostly more expensive (walnut & safflower oil) or don't cure ("dry"): Curing into a solid epoxy residue requires the oil to be on average at least doubly unsaturated. Sunflower oil doesn't meet this condition. – cbeleites Jun 18 at 8:34
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    Tung oil is another well-known drying oil. I'm not sure if it would work well for this application, and it's more expensive as far as I know. – R.. Jun 18 at 18:03
  • I often use pure tung oil for food grade wood finish. Takes longer to dry than boiled linseed oil–probably more like raw linseed oil–and is 3x more expensive, but it does not contain chemical metallic drying catalysts and is less prone to mildew according to the dealer (Spirit Line) and some other users. Kind of has a weird "fried food" smell. Worth a try on a small swatch at least. – llogan Jun 18 at 18:21
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    @llogan: over here (Germany) boiled linseed oil is available with (mostly called Leinölfirnis) or without polymerization catalysts and also varieties certified for food contact materials and children's toys. Price depends more on package size than on catalyst and food grade (hot pressed animal feed/food grade is ≈ 4 €/l -> 1,50 €/l for 1l -> 1t package size). – cbeleites Jun 19 at 15:53
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In addition to the answer by Charlie, Linseed oil is probably used in many instructions because it is what was historically used to create oil cloth.

A nice tutorial for creating historical oil cloth is for example the following YouTube video by an American reenactment channel:

Oil Cloth - Waterproof Coverings for Your Campsite

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