I've just renewed some of the putty on a window (on a very hot day) and some of it has cracks already. Does anyone know what's wrong and do I need to redo it? There are also cracks in a lot of the old putty (99% actually) but it seems to be stuck soundly. Do I need to renew this as well? Any help would be apprecoated.

Add alittle linseed oil before applying in this heat..

Or give the rebates a coat of primer first, this will help prevent the oil in the putty being soaked up too quickly.