Painting a building the right way is both a straight forward and complicated process. It is not too difficult to learn how to move a brush, but painting is a step by step process and a well done job can last for many years. A poor job will likely begin to peel and crack after a few years.
With a little knowledge, it is possible to finish the paint job right and enjoy the result for decades. If you are a professional, then it is always possible to learn ways to refine your product. Anything that is exposed to the elements needs to be well protected, which means multiple layers of paint and maybe a sealant to keep the moisture out.
The final coat will be the most visible, so it pays to do it as meticulously as possible. Previous coats such as the primer and the first layer of paint might be applied crudely because they will be covered by the final coat. If they are done unevenly, then it might still show as unevenness in subsequent layers.
The first step in finishing the paint job is to make sure there is enough layers to make sure the surface is well protected. Additional layers can also smooth out unevenness. Since paint layers cannot be practically removed, it might be necessary to add one or two more coats than you might have originally anticipated. This is better than a shoddy final product.
Having a thicker layer can be desirable if the surface was previously worn. Swaying branches and a very wet climate can have this effect. A thicker layer of paint will last longer and better protect against moisture. For outdoor structures that come in contact with the ground, paint can be a way to prevent wood from rotting.
The way you finish the job depends on the type of paint. Modern polymer paint will last without additional treatment. Natural paints do not withstand weathering as well and must often be coated with a clear chemical spray as the last process. In spite of the extra work, traditional paints can be worth the old time look and feel they bring.