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I'm waiting on the last coat of varnish to dry on a rod I just completed. I dipped it Friday night, and it looks pretty nice, if I do say so. No polishing needed on this one, whew!! I got to wondering, exactly how does varnish dry? I know that at first the solvents evaporate, but once it has "skinned over" and become tack free, what's happening in there under the outer surface? If I were to polish this rod, I would wait at least a week for it to harden thoroughly. But since it won't need polishing, and it's already fairly hard, is there any reason to wait before shipping it? (Harry Boyd) When I went through my apprenticeship many years ago we toured the Fuller O'Brien plant in So. Bend and we got to see and talk to many people about different things. Since I was using their varnish on rods I was restoring I asked them about their varnishes. I was told that varnish usually dries in a 24 hour period but it took a full month to fully cure at 70 degrees. I always have used this for a rule of thumb for letting my spar or other varnish dry. I am not sure about polys because I do not use nor like them. (Bret Reiter) If you used PU I would say that it is hard enough to go now. If you used spar, I'd wait your week. Spar will surface dry before the under part has polymerized, and it will stay soft longer. (Ralph Moon) What's going on is that the resins in the varnish are cross-linking to form a film by reacting with oxygen. This will continue well beyond the time it takes for varnish to "dry", that is, for the solvents to evaporate. As the finish crosslinks ("cures") more, it will become harder, which is better for polishing That is why most wood finishing books say to wait as long as you can before polishing the final the coat. Flexner's book says a good rule of thumb is a month, but you might get away with something shorter. I wouldn't go with anything shorter than a week, especially with spar, but that's just playing it safe. I don't know if heat and low humidity accelerate the curing process, but I bet it helps a little. (Sure reduces tack time, though). (Rich Margiotta) In talking about varnish drying, I separate the it into 2 chemical processes - drying to the touch, and curing. The "drying" takes place very quickly at the surface as the solvents evaporate, and the rod is dry to the touch and can be carefully handled. "Curing" is the chemical cross linking process occurring throughout the depth of the film, and takes far longer. Depending on the the of varnish, solvents system and drying agents, this can indeed be month. I've been using a simple trick to speed up the process - heat. Taking the rod to work, I leave it lying with the ferrules resting between the dash and seat of the car. It reaches some very warm temperatures, but not excessive (maybe in AZ in summer, but OK here in GA) and I can get complete cure in 3 days. Now have access to a lab oven large enough to accommodate rod sections and can control temp and humidity precisely. Will be experimenting with optimum conditions and report. (Carey Mitchell)
In talking about varnish drying, I separate the it into 2 chemical processes - drying to the touch, and curing. The "drying" takes place very quickly at the surface as the solvents evaporate, and the rod is dry to the touch and can be carefully handled. "Curing" is the chemical cross linking process occurring throughout the depth of the film, and takes far longer. Depending on the the of varnish, solvents system and drying agents, this can indeed be month. I've been using a simple trick to speed up the process - heat. Taking the rod to work, I leave it lying with the ferrules resting between the dash and seat of the car. It reaches some very warm temperatures, but not excessive (maybe in AZ in summer, but OK here in GA) and I can get complete cure in 3 days. Now have access to a lab oven large enough to accommodate rod sections and can control temp and humidity precisely. (Carey Mitchell)
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