I see down at the sheds you can get "deck sprayers" or "fence sprayers" (which appear to be the same apart from a slightly different nozzle). Apparently, they're intended to be used with deck/fence restorer sprays. Cuprinol are the famous one but B&Q also do their own one.
Anyone tried one of these ? Is there really much difference from the generic pump & spray thingies for weedkiller etc ?