Best paint?

[Jurek]

At least you're wrong consistently. 

It's my forte 

[Valiant]

There's always Tricoya which is mdf in steroids. Guaranteed outdoors standing in water for a bazillion years. And Valchromat which is lovely to work with, but it's pricier than even birch ply.

[mrcharly-YHT]

There also exists WBP ply, which is "water and boil proof" which may or may not be good enough to use as a marine ply. But it's what it says on the tin, it's ok to get it wet, or steam it without it coming apart.

Falls over laughing

WBP is theoretically durable outdoors.

In practice, if exposed to the elements (even when varnished), it delaminates in a few months.

If I were forced to build anything for external use outdoors, I'd coat with polyester resin (or epoxy).

[citoyen]

I only know about marine ply because it was recommended to me when I was making a new sign for our house, to stand at the end of the driveway. Finished it with yacht varnish (Ronseal, obvs). That must be 10 years ago at least. It has stood up to the elements very well.

Also used the yacht varnish to seal the edges of the MDF kitchen cupboard doors that I had to cut to size.

[Valiant]

There also exists WBP ply, which is "water and boil proof" which may or may not be good enough to use as a marine ply. But it's what it says on the tin, it's ok to get it wet, or steam it without it coming apart.

Falls over laughing

WBP is theoretically durable outdoors.

In practice, if exposed to the elements (even when varnished), it delaminates in a few months.

If I were forced to build anything for external use outdoors, I'd coat with polyester resin (or epoxy).

This, or use solid wood and let it breathe.