Moisture ingress
Cause
Old defective putties allowing moisture ingress into the timber resulting in the wood splitting and corrosion of the glazing pins.
Remedy
Hack out the old putties and replace after priming the rebates. Burn off remaining paint coatings followed by thoroughly sanding down, clean nail heads or replace.
Treat knots, etc. if present and prime overall. A general purpose primer suitable for wood and metal is usually recommended as this will contain rust inhibitive pigments.
This should be followed by an undercoat and finishing system oil or water based.