pressure-treated ramp to storage shed
The rule is - flick water on the dry plywood. If it beads, wait. If it absorbs, you're ready.

Always prime first. NOT Paint &amp; Primer-in-one hokey. That's not actually primer, it's just thick paint. Use a good quality acrylic wood primer rated for exterior, or better yet, use oil based Kilz.

Editting to add: I regularly cheat a paint job on green pt, using only acrylic latex primers amd paint. However, expect to have to repaint after one year.