This is a question I get asked frequently. It’s certainly logical that you would want to seal up your entire home as well as you can, especially in an important location like a deck where your family spends much of their time during the summer, where crucial load-bearing supports are open to the weather, and where the cost of replacement or repair if these surfaces are not maintained properly, is almost always very expensive!
However, counterintuitive to what most people would think, I would very strongly recommend not painting the ledger board and supporting joists underneath your deck in almost all cases. To understand why not, we need to have a clear idea of how water does its dirty work.
Next to the incredibly severe wind (and I’m thinking tornado or category 5 hurricane here), water is likely to have a more damaging effect on your home’s exterior than any other force of nature. You might not necessarily expect this. Particularly in semi-arid states such as Colorado where I live. Typically, around here, we all worry about what the sun will do to the finish on our cars, homes, hot tub covers, and many other finishes. But more often than not, the damaging effects of UV radiation on the outside of your home are easily repairable. What’s not so easy to repair is rot from moisture penetration. And while the strength of the sun’s rays is generally distributed evenly over a large surface area, water tends to follow a very narrow path time and time again, gradually eroding your home’s finish. Often this path is structurally speaking, an already weak path since it tends to be a corner, a groove, or some other tight space that is difficult for contractors to access in order to adequately seal them up.
And this is also true in the case of a deck. Think about this. A board technically has 6 edges that must be sealed to make it truly weather tight. The face, the backside, the left, and right edges, and the two cut ends. It is very rare that a deck installer will paint a board before it is installed. So the wood is put in place by the installer, and then sometime later a homeowner or an exterior painter will break out the can of paint and a brush and roller and go to work “sealing up” the new deck. Of course, the deck joists stand up vertically on one edge. So what you can access is going to be only the bottom edge facing down towards the ground, and the front and back large faces of the board. At this point, there is no way to seal up the rough ends or even the opposite edge of the board which faces up towards the sky and is now covered by the floor decking material which is screwed down on top of it. Even if you diligently paint the walkable surface of the deck, you really have no way to get between the cracks of the decking to adequately coat the upward-facing edge of the joists below. It really does not matter that you took the time to paint the other 3 sides of your deck joists. These get very little weather since they are mostly covered by the decking above. However, water must drain off the decking somehow. It all follows the path of least resistance and wraps around the decking to the underneath side of the decking boards and soaks into the joists below, through that edge that we have no way of sealing up. Over the years you can imagine the damaging effects.
Failing paint is the least of your concerns. I can remember one time where I poked a hole with my finger through a bubble of paint formed on the face of one of the joists and water came flooding out, like popping a balloon. The paint forms an excellent water-tight barrier. But, unfortunately, the water is just as likely to get trapped inside the wood by that layer of paint as it is to get stopped from entering from the outside. This is why it’s important that the natural materials of your home be allowed to “breathe” as the term goes.
So, while the unfinished and greying support structures of your deck are not visually very appealing, I hope this information causes you to rethink applying paint to them. If the deck sits at or near ground level why not consider installing a decorative lattice instead. Or if the deck is on the second level and is visible from below, maybe consider putting up a decorative fabric, some string lights, or some other visual distraction. You will be happy to have saved yourself the expense of a new deck down the line as well as the unnecessary work of a complicated paint job at present. And, in time, you will likely get used to seeing the exposed deck supports and not even notice them anymore. Happy painting!