Applying paint on a real wood deck is one of the two courses of action that can be taken to protect your deck wood from any damage. The other course of action is using a stain or sealer, which affords protection to the wood, as well as retaining the original color of the wood. However, it does not come with color choices, and is quite limited in terms of customization. That is why a deck paint is usually preferred over using a wood stain.
When choosing a suitable paint to apply on your deck wood, there are a number of factors that you have to look out for. The paint should be easy to apply, quick to dry, and easy to remove should the need arise. There should be suitable color choices that have a good texture and finish. The paint should provide appropriate protection to the wood from all environmental influences like moisture, UV rays, rust, and mold. These factors are greatly affected by the type of wood paint you are using. It can be water acrylic based or oil based. I will let you how the base affects each factor.
The durability is an important factor for the reason that the deck is for rough usage. The water acrylic based paints are comparatively less durable than the oil ones, to be honest. They only cover the exterior, and do not possess the ability to penetrate the wood. They do impart adequate protection to the wood, but tend to chip away after some time. This is because they are somewhat susceptible to the rays of the sun. An important example is the Rust-Oleum latex, which has succeeded in removing this handicap attributed to the water acrylic paints.
The oil based paints have the ability to penetrate the wood, hence, imparting greater overall protection to the wood from external influences. However, their color tends to fade away with time, and peel off after the repetitive washings. But products like the Diamond Brite do retain their color for at least a year, and provide very good protection against all the external agents which come in contact with the deck. The wonder products like KILZ manage to bring the best out of both types, so they are also something to look for.