Nessmuck: Truly, I'm a novice at painting. Sometimes I thin, sometimes I don't. And I'm terrible about reading manufacturer's recommendations. Best advice I can give you is apply the paint as thinly as possible as you'll end up with fewer drips/streaks. It's tempting to think that thick coats fill voids quicker, but they can drip something awful and take forever to dry. I thin when I find I can't get thin coats or if it takes a lot of work to get complete coverage. Thinning helps you get thinner coats, but with the paint being less viscous it can also be prone to dripping. I also always sand between painting. I figure sanding helps contain drips - it's like having little ridges to catch drips if you sand left-to-right. O, I sand about 24 hours after painting then let it sit for another day before I repaint. I figure by sanding I'm exposing paint inside the layer that hasn't see the air yet, so I wait to let it dry too.
O - most of my canoes have keels. So what I do is prop the canoe on its side at like a 30 degree angle (I rest an inner rail against my trash container) and paint only one side from the keel down. Then in a few hours when the paint is tacky enough not to drip I repeat on the other side. I figure that way the paint by the rails (the part most people see when the canoe is in the water) is on a less steep surface and gravity has less of an effect. It also makes it easier for me to see when I'm painting.
And kill all the #$%@ flying bugs before you paint! And don't paint on a windy dusty day!