DIYbanter - Painting a roughcast house on Isle of Lewis

I'd be grateful for advice on painting the currently unpainted
roughcasting (pebbledash) on the outside walls of my 40-year-old house on
the Isle of Lewis in Scotland. The wind is often strong here and rain
can be heavy (and salty), but temperatures rarely if ever drop lower than
a few degrees below zero Celsius. I need a long-lasting finish,
preferably a very long-lasting one. (So I will not be using Dulux
Weathershield!)

At the moment in places the roughcasting visibly holds moisture on wet
days which evaporates on dry days.

With the (flat masonry) external window sills and surrounds I have had
good results using

* a coat of Zinsser Bulls Eye 1-2-3

* two coats of a generic "white label" pliolite on the surrounds and
three on the sills.

My results have been better than the results a contractor got using just
two coats of the same pliolite, without any undercoat, on the coping on
the local church's wall, where the paint has already started to peel. The
comparison is not conclusive since there may be a guano factor, but it is
well known on the island that many contractors will swear blind that such
and such a way of doing something is best because they like to use cheap
materials for the same job price and also because if they didn't get
maintenance contracts after a few years they wouldn't get much work.
Nobody punches them on the nose when they come across all "honest" like
little boys who've just thrown stones through a window and are denying
it, because they're probably their uncle's auntie's best friend.

However, the roughcasting is different from the window surrounds because
it is stained in places, on wet days it visibly holds moisture, and it's
knobbly.

So I was thinking as follows.

* a coat of SBR, followed by

* a coat of Zinsser Bulls Eye 1-2-3, followed by

* two coats of pliolite (e.g. Zinsser's "AllWeather" but another
manufacturer's might be just as good or superior).

I should say that I am a fan of the 1-2-3 which I've found is a superb
undercoat in several applications and gives much better results e.g.
applied to partially rusted metal with a standard gloss then put on top
of it than a dedicated product such as Hammerite.

Another option might be to use a masonry gloss, such as say Zinsser's
(water-based) Allcoat Exterior Gloss, instead of the pliolite. I mean
there must be a reason why lighthouses which take an almighty battering
always use gloss paint.

Or rather than using SBR and 1-2-3 I could use only one of them.

Or I could dispense with both and use only two coats of pliolite, with
the first diluted slightly with white spirit.

All advice will be welcome!

Thanks in advance,
Harold