On Fri, 22 May 2020 10:37:31 +0100, Scott
wrote:

I have noticed some of the paint on the outside of my window frames is
starting to blister. I remember when the windows were installed they
made a thing using about using microporous paint.

Does this mean I need to use microporous paint again?

You don't have to but it might be a good idea (less chance of a
reaction and provide good / breathable protection).

If so, can I buy microporous primer, and is this needed?

Probably but no.

Could or should I just rub it down a bit and use exterior paint?

No, if you are going to use a non-porous paint you might need to go
though the std staged or use a good one coat solution.

Is water-based paint any good for outdoor use these days or should I
be looking for oil-based paint?

Depends if you want breathable or not.

It may well be that they were originally painted with a Sadolin
product:

https://www.sadolin.co.uk/?s=Sadolin+Superdec

Could I use the paint Network Rail used on the Forth Bridge :-)

If you want it to look like a rail bridge. ;-)

Our front picket fence is painted with white Superdec and has only
been painted twice in over 20 years. The last time it was by daughter
and she found it very easy to prep for and paint. Being water soluble
is also good for cleanup and the environment.

Unlike gloss paint, it goes on very easily and is less fussy re
brushstrokes etc.

Cheers, T i m