Thanks for replying,
Looking at it closely, I believe the yellow parts are wood, as is the frame etc. Does that change things?
Ok, I assumed that it was uPVC because exterior gloss seldom yellows. It is a lack of ultraviolet light that causes paints to yellow. Not normally an issue outdoors.
At the risk of offending your powers of deduction (which is honestly not my intention), have you tried to sand a small section of it or used your fingernail to see if you can leave a slight dent.
I do not recall ever seeing a metal skinned door with timber glazing beads. Then again though, I have only worked in a handful of houses with metal composite doors.
If it is timber, you can pretty much use any exterior wood/metal paint. I have never used the Zinsser paint that you linked to. I did see someone else use it on their place this year and will watch it over time to see how it stands up. Hopefully other members will be able to give you their experience of using it.
My biggest reservation is that they only guarantee it for 2 years. Dulux Trade oil based gloss is rated at 8 years (but with no definitive guarantee).
From the Q&As on the
screwfix site, someone else asks about painting metal, the response from the Zinsser rep is-
"Hi. Thank you for your recent enquiry
Yes, the All Coat would be suitable for application to a previously painted surface. If the metal is powder coated you will need to lightly abrade the surface to aid the key of the All Coat Our best recommendation would be to ensure that the surface has been prepared sufficiently i.e., with hot soapy water then rinsed with fresh warm water to remove detergent and when dry a good wipe with methylated spirit. 2 coats are recommended to be applied by a brush or roller and allowing 1 hour between coats".