Before you paint your house, you should consider whether the current coating is a paint or a stain. This can make a very big difference in the approach to your prep work. The first step of determining if your house is painted or stained is a visual examination of the material being painted. If the house has a transparent coat over wood, it is a transparent stain. If the surface appears to have a solid color it could be either paint or stain.
Stains primary purpose is to soak into the pores of the wood surface. This will give you a visual cue. Wood that is stained will keep the look of wood, even with solid color stains. If you can see the wood grains and can feel the wood surface, it is most likely a solid body stain. If the wood is smooth and all of the wood grain is filled in, it can be a good indicator that the surface has been painted.
However, if over the years the stain hasn't been able to break down and fade or chip off, the three to how many ever coats applied can give the appearance of paint. This is because the wood has been fully saturated by the solid body stain and is not allowing the other coats to penetrate the wood. This causes the stain to form in layers on top of the wood.
The next indicator of a coating is how it fails. Generally, if the surface is painted, it will peel off in sheets or long strips. Stain tends to fail in the form of fading and chipping. When a stain chips, there will be small square or rectangle pieces chipping off of the surface.
With these two indicators, you should be able to tell if the surface is painted or stained. If you cannot tell, make sure to have a paint representative take a look for you to make sure. Once you have decided what you are going to repaint with, you will need to prep differently. If it is a stain, you should not prime bare surfaces as the functionality of stain, as mentioned above, is to penetrate the wood grain. Applying primer will deny the stains ability to soak in.