Before you start painting, it’s imperative to prep the shiplap board. Many types of real shiplap boards come pre-primed, saving you time in this phase since you won’t need to prime!
You can start by ensuring none of your nail heads are sticking out of the boards. Next, use a putty knife to fill every nail hole, seam (between panels), and knot in the wood.
Take 100 to 150 grit sandpaper and sand the entirety so the grain is completely smooth. Then use a rag and vacuum to clean the dust off the shiplap.
Once everything is dust-free, check for noticeable shiplap cracks that must be sealed with caulking. Take your paintable caulk and fill in all the nail holes, cracks, and seams. You’ll need to remove any excess caulk with a rag. Let the caulk dry completely before painting the wall.