Dulux Trade Weathershield can suffer "plastic bag holding water" and is not over coatable (burn off).
Sikkens Rubbol AZ does not suffer the "plastic water bag" effect, does not flake, can be overpainted. However you need a sound surface because it can be intolerant and it does not like fresh putty (shells off). If you paint fresh hardwood which has high oil content wipe it with methylated spirits first (eg, Iroko).
Beware the high end Rubol (BZ or CZ or SZ) because they require kiln dried fresh wood to get their huge life.
International (Akzo Nobel) 10yr gloss is well regarded, it outlasted weather shield with ease.
Weathershield has an inferior gloss to Rubbol AZ year 1, but by year 4 Rubbol Az can look a little chalky (which is how it wears). Beware Rubbol is only removeable with mechanical means - a bit like Sikkaflex
225 on your fingers in that respect. Decorating Direct used to do Rubbol, the colour chart is hard to get hold of re obscure Sikkens codes... you might get the Sikkens / Dulux Website to deliver for =A35 without getting a dented tin like some suppliers. Akzo Nobel tins seem to be round, ICI tins notorious for not being (plastic bag or store upside down or kilner jar).
Microporous paint... well the water does get UNDER Dulux weathershield after a few years :-)