linseed oil putty

Would appreciate advice on choice of suitable primer... I painted a Crittalls window (i.e. zinc galvanised steel) last year using this Rustins water-based paint as a primer: https://www.rustins.ltd/rustins/our-products/speciality-paints/quick-dry-galvanised-metal-primer But I didn't like the...

I have softwood single glazed window frames ( about 25 years old ) in a very punishing south facing elevation and the putty shrinks and cracks a lot, on the bottom bead particularly, and needs replacing. I would be grateful for any thoughts as to whether linseed oil putty is the best replacement...