I have made or repaired a lot of window sash, and while I’m pretty good with a putty knife, I still find that glazing takes an inordinate amount of time. One of the minor frustrations is that every can of glazing compound has a slightly different consistency, while there is only one consistency that makes puttying easy, fast and even remotely enjoyable. Even within cans of glazing compounds there are differences between the stuff under the lid and the stuff on the bottom.
I used to add a little bit of paint thinner and mix it in by kneading, but now I’ve changed to adding just a few drops of Penetrol (H. P. Flood Co, www.flood.com), an oil-based paint additive. This seems to make glazing compound much more plastic and adhesive, not just softer. Thumbing on a bedding bead is a breeze, and I can load great gobs of putty on the knife and apply it in one long stroke for my exterior bead. Remember though, only a few drops to a fist-sized ball of compound
Daniel Wing, Corinth, VT