That moving around a bit is a problem, however. Swelling and shrinking eventually lead to cracking and splitting, which is why if you have a wooden structure that will be exposed to the elements, it needs to have some sort of protection. Some woods, like teak, redwood, and cedar have this protection naturally. They'll weather nicely, turning gray, but otherwise showing little wear for years and years.
Other woods do not fair as well. The mighty oak, for example, that majestic tree that we all like to compare ourselves to when we are pretending to be tougher than we actually are, is a real wimp when water comes creeping around.
And pine? Forget it. It folds faster than an origami chair.
So if you are faced with using a wood that is not cedar, redwood, or teak for an outdoor project, you need to offer it some protection from the elements. In most cases, this means painting. Since restoring a previously painted structure is harder than painting a new one, I'm going to focus on that. Here is what I have learned from many, many, many years of a) not listening to my dad, b) deciding finally to listen to my dad because doing a) turned out not to work so well, and c) experience as a poeticky woodworker who is also a realist.
1) Don't take short cuts. Prepping is boring, it's hard, it's time-consuming, and it has zero instant gratification. Nevertheless, if you think, "Oh for goodness sake, how serious could it be to leave a little loose paint or dirt on the surface?" or "I'll just fill in that little crack with some extra paint," put down your tools, walk away, and go buy a new gate, because the paint job you are putting on this one is going to fall apart as soon as you turn your back. In fact, the moment you start to say, "How necessary is..." this paint job is going to start looking around for a place to die.
2) Remove ALL loose paint. Then sand the edges.
3) Fill ALL cracks and holes with good, fresh, flexible, paintable wood putty. Don't be seduced into thinking that you can just put more paint in that crack and have it hold together. Modern paints are fabulous, but they can't work miracles.
Here is why you are filling all the cracks: most paints have some flex in them, which is what you want because the fibers of the wood are squirming around slightly as the moisture content of the air is changing, remember? However, they have limits on how far they can stretch, so if you don't fill the cracks, eventually the surface of the paint will fail. Once the surface of the paint is breached and a hole appears, water will seep in and the wood will swell even more. It is over at that point. You have lost the battle. Let the paint peeling begin.
4) Remove all dry rot. There is a reason it is called "rot." It will not stop once it has started, no matter how many layers of paint you slap on there. Cut it out until you see fresh wood. If this makes a hole in your sill, or removes part of a panel, replace that with fresh wood. If the piece is small enough, you can even use bondo--that putty used for repairing dents in cars.
5) If you need to glue something, as I did with the joints in the gate (but not the slats, since you actually want them to be able to move around as they shrink and swell; if they are unable to do this, the wood will split), use a quality waterproof glue. I use Titebond III (not Titebond II, which is only water resistent). This is the glue I use on my canoe paddles, and it holds up to water wonderfully.
This is the take-home message for prepping: While some moisture will always be in the wood, the purpose of the paint is to create a protective skin to keep as much water as possible out, all in order to limit how much movement occurs. You have to do everything you can to keep that protective layer intact. Hence the scraping, the cleaning of dirt, the filling of holes.
1) Prime all surfaces, even if you've got the fancy paint that claims to be paint and primer all in one. Trust me on this. Remember, you are trying to seal the wood, and this is what a dedicated primer does best.
2) Use two coats of paints, even if you've spent the big bucks (which I recommend on paint) and gotten the high end, so-called "one coat" paint. The advertisers lie. You will need two coats.
3) Between coats of paint, you can wrap your paint brush tightly in a plastic bag. The sky will not fall down if you don't clean it between coats, and this saves you some labor. You worked hard on the prepping, so you are due for some labor-saving.
4) Use a paint key to open your cans of paint, not a screw driver. The store where you buy you paint should give you these for free. If they do not, go someplace else. Screw drivers ruin the edges of the lid, making it hard to close it tightly.
5) Keep the rims of your paint can clean to ensure proper closure of the lid. There are a variety of ways to do this. If I have to pour the paint into another container, I use a special plastic lid with a hole in the top that keeps the rim clean. You could also simply wipe it with a paper towel.
6) Use a large rubber mallet to GENTLY tap the lid back down on the can. Don't use a regular hammer (SEE ABOVE, in re damage to the lid).
6) When you are finished, clean your brushes until you see no more paint coming out of them.
Now you are finished and you have done a fabulous job. Your gate looks adorable. Even so, it is still an organic thing, so don't expect that this happiness will last forever. Eventually, you will have to go through the process again. The good news is that if you have done it right this time, that moment will be a long time coming.