I bought a sprayer last year and it works fine, it will do around 20 fence panels (then you must change the batteries) I did use a brush after the spray went on because I found that if the timber is too rough the coverage wasnt too good, so a quick brush to tidy things up, but generally the sparyer was fast and neat (spary in very calm weather)
also, I notice that a 5ltr drum of the sprayable 'stuff' is around €14.99 whereas the 5ltr ordinary brush 'stuff' is around €8.99
Hope this helps