My one sop to modern materials is when fitting windows. I tend to shy away from using conventional putty on external and opening units, using a silicone based sealant instead. The trouble with putty is that it sets rock hard, which transmits shock and movement directly to the glass, and it does not weather well and will crack over time. The first two photos below show a wooden frame and a cast iron frame, both where the putty has failed. In the wood frame water ingress has rotted the frame, and you can see rust in the iron frame. If left untreated the rust will blister and crack the 200 year old glass.
Silicone on the other hand forms a permanent (but easily removable should you need to) waterproof bond, which remains flexible enough to absorb shock and movement. The bond is strong enough to hold the glass by itself, but I always use a second mechanical fixing as a failsafe measure. Copper tacks are best as they can easily be bent onto the glass/lead to secure it and will not corrode as easily in damp timber or react with the tannins in oak.
