Using an exterior wood preservative prior to painting will help to protect the wood from mould, algae and insect attack. When selecting a wood preserver however, be sure to pick one that is wax, oil and silicon free. Preservers that contain any of these elements may prevent or reduce the ability of the paint to bond with the surface of the summer-house. For this reason we always recommend doing a test area before starting any project.
Exterior wood preservers that contain wax or oil are fine to use if the summerhouse is going to be oiled rather than painted.