Even if you haven't added any pigment to it, you want to make sure that the entire has an even distribution of resin. Resin is what helps seal the porch surface and provide a sticky foundation for the paint. Pour the primer into your paint tray, but before you saturate your roller in it, dampen the roller with water. This helps "prime" the roller to absorb more primer and helps provide an even application to the porch. When applying the primer, use a thin coat and only apply it to the areas where there is bare wood. Use your brush to make sure that you get in between the boards and into every crevice you can see.
For this step, we recommend Zinsser 1-2-3 Primer. It's water-based, so it's easy to use and clean up. It's also mold and mildew resistant. It dries quickly and is ready to paint in an hour. This primer is approved for both indoor and outdoor use, so it's a great primer to keep on hand.