Prime first as the blue will be a challenge to cover. Get a small brush. It will be effort-full but should paint easily. Maybe you could have just sprayed the head boards you have.

Instead of painting the ship lap solid glaze it lighter on each coat. Will add character and interest.

Use a brush for cracks, just make sure brush marks aren't too noticable on facing - they will show when you roll the rest of it. Unless you 2 people working on it - one brushes, one rolls. Use a primer for color changes, probably do two coats since it's a small area. I did Brave Purple a few months back, same area as you - behind headboard.

KILZ or Zinzer make good primers. May take 2 coats though before you can repaint.

wow, I love it when commentors don't answer the OP's question, and instead insult their design choices. helpful.

flboomer46 yes you will have to brush the seams first. it's tedious but you might be able to get away with doing that part only once. my guess? one coat of white primer over the blue paint may hide it just enough so that it reads as shadows in those deep seams. brush paint once, then do two roller coats in a test area. stand back and see if any blues stand out. (be sure to look at different times of day in different lights.)

I recently painted a cinder block wall that had really deep texture. I was going from a light to dark color but still only needed one coat for the brushed parts. I cracked open a beer, put on my favorite tv reruns in the background, and just turned it into a party.

Paint it in the late winter (now), when the air is more dry and the wood has shrunk. If you wait until summer, after it's swollen, you'll see blue strips next winter.

I would also paint the grooves with one coat of thinned white paint--work it in well--which can spread through capillary action up into the tongue/groove joint. It won't be as solid, but it will be less noticeable if it shrinks.

Plan for touchups as the seasons change.

Oliviag/ bring back Sophie I was talking about any paint job on something that has lots of deep crevices (it's an old cinder block wall and the mortar was super rough). I was pleasantly surprised that I only needed to do one coat with the brush, since that would be the worst / most time consuming part.

B F - self-righteous much? My post was to try to save the OP from a very tedious job of re-painting, and which won’t result in improvement to the issue of the room appearing smaller (and more squat due to the horizontal lines). My opinion about shiplap is that it was a flash in the pan fad, that often was applied to areas to which it wasn’t suited. At least I am trying to give the OP another viewpoint to think about.

Assuming that you primed before you painted the wall blue? And used BIN (shellac) over any knots?...I would paint over the blue paint with Zinsser's 123 primer. Easy to find, not expensive, decent hide.

Yes, paint it now, in the dead of the winter. Or wait until next winter if you just installed it and this is real wood shiplap because it will continue to shrink.

I would just brush all of it. Or brush the cracks, roll and back brush as you go. Work fast. Because modern paints dry so quickly, you can't brush the cracks and come back 15 minutes later with a roller.

Because of the gaps/ cracks is why I think just brushing (and forgetting the roller) would work best to avoid get too much paint gunked in the cracks.

IMO, the room will feel larger after it is painted white. The ceiling height is what it is but the high contrast between the blue wall and white ceiling will make it seem lower. As well as the horizontal lines. Eliminating the blue will help a lot.

BTW, I love exterior siding used on interior walls. I fell in love with it over 30 years ago when I saw it in some friends house...where the house had been added onto and the original exterior lap siding had been left in place and was now an interior finish. I copied the idea just a few years later when we remodeled a bathroom. Rather unique at the time.

I have a guest bedroom with walls painted blue, though a lighter color than in the OP. It’s BM Beacon Gray (flat finish). The room faces north with trees over it, so not much light coming in at all. The blue actually makes it look more bright and warmer than the previous cream. I am imagining my guest room with shiplap like the OP’s, and when I picture that in my mind, it would seem to make the room more closed in. Therefore my advice to the OP is to choose a paler blue, but also to be honest, remove the shiplap if you can. Here are a couple pics (not mine) of BM Beacon Gray, on drywall.