Finding a Durable Material for a Picnic Table

As stated above, use deck stain and I always make sure it's OIL-BASED water-proof stain. As Lil Tigga above says, any hardware or big box store will help you with this.

Use a Stain not a paint. Pressure treated wood shouldn't be painted or you have to always do it. The paint will always peel off. Any hardware or big box store will be able to help you.
Good Luck!