arghhh! blasphemy!!! i am a window restorer by trade and bristle at the suggestion that replacing old windows is a good thing....
the best practice for repair of cracked glazing putty is, of course, to remove ALL the putty, glass and putty bedding....alas customers may not want to (or cannot afford to)go to that expense...may i suggest that you: remove crumbling putty that comes out easily, prime remaining area with cover stain, apply putty*, wait a week*, prime with cover stain and paint as usual...
*aqua glaze is difficult to work with--there's a learning curve--but can be painted in a couple of hours...the label says 'sterling-clark' but the product is now made and distributed by savogran..
*UGL glazol is similar to old traditional putties with added chemicals for drying and bonding...the manufacturer suggests it can be primed with an oil based primer in 24 hours and water based primer in 7 days....i waited three days, primed with oil, 2 finish coats of acrylic paint and the oils 'burped' the paint right off...
*many window restorers use a product called 'sarco, type m'...it is very much like traditional putty, skins over in 3-4 days for priming...it's only available through the manufacturer in chicago at 800-969-7889...though they advertise 1,2,3,4 & 5 gallon containers, they usually only have 4 &5's available...
old wood windows rock!!
...jade