Turning Archive 2005
>It is that time of the year again, and everyone is starting to have problems with their oil finishes.
Start by reading the directions on the can, and what it says about hot and cold temperatures, and wet weather. These directions will tell you that if the sweat is dripping off the end of your nose, you have to wear a coat while working in the shop, or if it is raining outside, you are going to have problems with oil finishes.
Oils cure by a process of polymerization where the smaller liquid molecules link together to form a larger solid molecule that we call a "finish". The thinners in the oils give them a pool so they can swim around and get linked together. In the ideal conditions, the solvents evaporate as this linking takes place, and all is well. We use the word "dry" to describe what is happening.
This process generates heat that is carried away as the thinners evaporate. Left to dry in a confined space this heat can reach a temperature that will cause cause combustion of an oily rag.
All oil finishes work the best when the temperature is 70F and the relative humidity is 70%. At higher temperatures, the thinners can evaporate too fast, and the molecules of the oil never get a chance to link together to form a finish.
At the lower temperatures, the finish and the wood is too cold for the heat to generate. No heat, no polymerization.
Without making any adjustments for humidity, you can figure that the drying time will double for each 5 degrees of temperature below 70F, and that it might never cure at temperatures below about 45F. That means that the finish that dried overnight (less than 12 hours) at 70F can take 24 hours at 65F, 2 full days at 60F, 4 days at 55F, a week at 50F, and you will be lucky if it dries at all below that.
There are 2 solutions to the cold weather problem. One is to make sure that the oil and the wood are both at room temperature when they are applied, and that they stay at that temperature while the finish is drying. The other is waiting until spring and warmer temperatures to use oil finishes.