beware of the boil over that can occur when adding the oil to the resins. I asked george about this, because i had to keep removing the pan from the heat to keep it from boiling over. I don't think my burner would ignite it, but it's possible and the way it foams up, I'm sure it would be impressive. (george used the term mushroom cloud for batches he's made that have gotten out of control).
It took me a long time to get the varnish to temp because I didn't want to cover it and let it get out of control. I eventually covered it to get it to 300C because that's the only way my hot plate would do it.
I would advise that as a strategy actually, if you want a dark amber varnish without it being so dark as the pigmented types are (they can look unnatural). I probably had my varnish at 260C for an hour or two where it slowly got to that and stalled, and kept string testing it. It needs that last bit of temperature for the resins and oils to bond properly (or whatever is happening chemically).
It's been a real treat to use.
As an aside, george mentioned that he has mixed the resins and oils together from the start and had good batches from it. I'm not sure if it would still boil over. If not for the fire risk the stink coming off of the oil as it heats (smells like you're cooking something, but it's still smoke from oil that's past the smokepoint, so I'm sure it's not good for anyone). Even so, he said something along the lines of cooking it in a tall narrow cylinder where the cooked amount is only a small volume of the cylinder, so there must still be potential for boilover.
Sorry to be long winded. the final varnish hardens in a couple of weeks if you just let something hang, or it hardens 90% in a matter of hours in the sun, and then can slowly finish over time. It's a pleasure to use - I added about another 75 -100 ml of turpentine after the varnish was finished so that it would be easier to wipe, and even at that, it's still about 30% solids and covers and builds very quickly. I'd anticipate the batch to have made about 20 ounces of varnish, which would easily cover a couple of pieces of furniture, and I'm thinking maybe 10 or so guitars - just guessing. Much less sticky than epifanes when it comes to getting off anything that's accidentally gotten on your hands, and nicer to wipe with because the lack of sticky seems to make it easier to pad onto something.
The suggestion of the space bags makes it really easy to preserve so far - only a tiny bit of varnish at the tip of the bag is exposed to air, so you'd lose a gram or three if anything, and that's it.