Paint Expert here.
You will only need a quart of each color to get started. Or whatever the equivalent is
in Eire. You can always go back and get another quart of each. Six big gallonish containers will cost a lot – maybe don’t make that big commitment till you see how it goes.
Make sure the weather has been very dry for some days – if that is an option. If not,
the milk paint will be great, because as a water-based paint it will dry quickly and will bond to any residual dampness in the boards, instead of being resisted by it.
(Resistance between oily and watery = peeling.)
For a very weatherproof surface, I strongly recommend Porch and Floor Enamel – again, if there is such a thing in Ireland. This
is very very strong wear-resistant paint that’s designed to be walked on.
I use it for all glossy trim work because it simply never chips or scuffs and can be scrubbed. You can have them mix it for you in your gorgeous colors. You’re not limited to color chips.
American paint companies, and maybe European paint companies, make both oil-based and latex-based floor and porch paint. Latex-base might be a good bet for you, because although not as impermeable as oil-based, it’s a whiz to use (soap & water cleanup), it’s pretty non-toxic (no hideous petroleum-based solvent), and you could slap it on the boards even if they’re not bone dry. But the drier the better.
A last note: some people really like a little mossy look to their wooden outdoor
construction. Milk paint is famous for hosting little moss colonies because the milk rots slightly while settling and provides nice protein for their dining pleasure. With any paint, however, you can put a little bit of moss and some regular milk in a blender, then splash or paint the mix onto the wood in misty weather.
It will STINK for a bit (that’s the organisms working) but soon it will look fab.
I love painting so much. I wish I were in Ireland to do the Tom Sawyer’s Neighbor
Boy thing for you.