So my northern California shop temp is hovering between 40 and 65 degrees Fahrenheit this time of year. This is too cool for oil-based varnish to cure. So I was wondering if thinning the varnish and applying thin, wipe-on coats each day would allow for a full cure.
Drying yes, curing not really. The cure rate won't be greatly affected (because you can take it that the curing doesn't begin until the final coat has gone on) but you will most definitely see a difference between brushing on a varnish and wiping on three or four thinned coats of the same varnish.
I almost always apply varnish by thinning it and wiping it on and by doing this I consistently get varnishes (more than one type) to dry and cure through most of the cooler months at room temps that are sometimes below 10°C, which translates to under 50°F.
As you might already know, using wiping varnish you can wipe away all, some or none of the excess after application. I highly recommend the first option in this context.
When you wipe the surface very well after application you build the finish and develop sheen much more slowly, but drying is far more reliable. If I wipe away all excess (the surface is virtually dry) the project is ready for recoating in 12-24 hours but if I leave a thicker film by lightly wiping the surface it can jump to in excess of 48 hours.
Note: once the temperature drops to closer to 2°C (36°F) I prefer not to risk most finishing operations as I've tried it a few times and it hasn't worked well. The problems I've experienced include really excessive varnish drying times — two weeks to become tack-free — but other other more serious effects are possible (finish defects) so it's best not to take the chance with most final finishes.
Please note any other readers, this should not be taken as a recommendation to finish at low temperatures. As much as possible finishing should be done in a "warm, dry, dust-free environment". Warm here should be taken to mean average room temperature or higher. Only by following this can consistently good results be achieved using a range of finishes.