Preparation for painting and decorating new doors and windows
New wooden windows and doors may already have a coat of pink primer applied at the factory, but it’s best not to rely on this for complete protection. Knots, for instance, will rarely have been properly treated, and the primer film will have been damaged here and there in transit.
So sand down the whole surface, treat any knots with knotting compound and apply another coat of wood primer overall. It may be advisable to paint doors while they’re lying flat; certainly it’s vital to paint the top and bottom edges before you hang them in place. It’s very important to paint the bottom as rain and snow can easily penetrate unpainted wood causing it to swell and rot. Paint also protects the wood against attack from woodworm.