The problem is that
they say to use a solvent based microporous paint but I couldn't source it
anywhere. I tried B&Q, Screwfix and Ridgeons without any luck :-(
I'm not surprised. When these paints were all the rage in the 70s, it was
acknowledged that each coat reduced the microporosity, and a single heavy
coat did the same. At what point the paint ceased to be at all microporous,
nobody ever quite established.
If you want to play Baltic's game, International used to do a solvent based
version, but it was crap so I imagine they might have ditched it by now.
Anything made of softwood that faces south/ south west will have a limited
life, whatever you treat it with. I would go with an oil based paint
(eggshell if you don't like gloss). One undercoat and one topcoat on top of
your existing coat will give you the best chance.