We don't use latex at all, we use Ronan or Chromatic bulletin colors and some one shot. We can spray, roll or fill paint. The primers the guys were using was still allowing a rough look to finish so now we've switched to blockout white from Chromatic thinking it has more pigment than the previous. I'll let you know how it goes once we finish some. Is there a name brand for marine enamels and is it available from many suppliers?
XIM is what I have used in the past for steel sign poles its a good product, I don't recall ever using it on wood though. We use fast dry blockout white from Chromatic now. The guys were using something else prior and it was not taking care of the fuzzy surface MDF has especially after routing so I asked them to switch to blockout, the pigment is heavier in this verses what they were using. I'm still against using MDF but its not my decision to make, I'm just concerned we'll have some come back because of water damage.
MDF is not good for outdoor uses.
For paints, a good quality enamel primer and One Shot paints are best in my opinion. As a cheaper alternative you can also use One Shot's Graphic Coat line of bulletin colors.
Ronan and Chromatic are good paints as well .. But i find One Shot paints cover best.