What to mix with window putty to prevent cracking?

The biggest reason putty cracks is exposure. Window putty needs to stay painted with the window. All putty dries out and cracks over time, but lasts,a lot longer if protected with paint.

Your putty should be of a type for your region's climate, and indicated for window use. Newer putty products far exceed the old ones for durability.

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I assume you are talking about window glazing compound. Never use caulk on a window! Glazing will always crack with time so keep it painted. Old window sash is generally quite dried out so I always pretreat the mullions with boiled linseed oil. This is typically the same oil used in the glazing compound so this helps keep the oil from being sucked out of the compound too quickly and causing immediate cracking. I own a Victotian house and i think iust have reglazed them all. This works for me.