On pine boards, I have successfully used this bed-board painting technique from Mar-K:

Quoting Mar-K:

This coating has not failed.
Only slight dulling of the original gloss.
No other defects.

If a weather resistant finish is desired, this is the best system we have tested.

Oak boards [I have also used this on pine boards, TCL]:

Coat all six sides of boards with three brushed-coats of POR-15 Rust Preventative Paint, three hours apart.
then
Coat all six sides of boards with three brushed-coats of Behr best quality black [I used semi-gloss, TCL] exterior latex enamel, three hours apart


Original posting by Mar-K
POR-15 Black Rust Preventative Paint and BEHR Gloss Black exterior paint