Finishing Oils and Temperature
Russ Fairfield
>This is the time of the year when we recognize our finishing sins of the past winter. It is also a difficult time of the year for oil finishes because of lower temperatures at night. And we are approaching those days when it will be too hot and the humidity too high for using an oil finish.
Oil finishes are sensitive to temperature and humidity. Every can of oil finish has a warning on the back of the can label. If it doesn't, it should. Watco prints it in bold letters. It goes something like this: "Do not use this product in extremely cold, extremely hot, or wet weather".
I read that as saying that IF you have to wear a coat in the shop, or there is sweat dripping from the end of your nose, or it is raining outside, or there is dew on the grass, you shouldn't be trying to us an oil finish.
The reason is that, what we call the "drying" of the oil is really a polymerization of the smaller molecules of a liquid into the larger molecules of a solid. The thinners are there to facilitite this reaction, giving the oil molecules something to swim around in until they can link up. Idealy the thinner evaporates after this linking of the oil has started to take place. If it evaporates too fast (as in hot weather) the oil molecules never get together and you have a sticky mess. If the thinner evaporates too slowly (cold or wet), the molecules give up, and you will have a sticky mess.
The polymerization is an exothermic reaction where heat is generated. If it is too cold for that heat to develop, nothing will happen. The best temperatures for an oil finish are between 70F and 85F. Higher than that and the polymerization can take place too quickly. Lower than that and the molecules will get sluggish. Below 65F, they start slowing down and it may take several days for the oil to "dry", if the thinner doesn't evaporate first. At 55F they are getting very slow, and by 45F they have stopped altogether. These are general temperatures because humidity and rate of evaporation of the thinner is also important.
Japan Dryer is a bandaid ingredient, not a magic solution for an oil finish the won't "dry". It can be used to hasten the process within the temperature range. I use it at the lower end of the temperature range, 55F to 65F as the nightime shop temperatures. Below about 55F, it is too cold for even the Japan Dryer to save us from something we shouldn't be doing. And, the dryer will not save an old oil finish that has started to polymerize in the can. Old oil will polymerize more slowly, if at all; and adding Japan Dryer will not save it.
The question after the finish has failed to "dry" is always how to save it, or how to get it off. If you get to it quickly, a sticky oil can sometimes be saved with a thin wipe-on coat of the Japan Dryer. If not, it will be necessary to remove it. Sometimes it can be removed with a thinner, mineral spirits or turpentine. If it is too far along for the thinner, we will have to wet sand with coarse sandpaper and useing mineral spirits or turpentine as the lubricant. Sometimes a fresh oil finish can be used for the wet sanding.
