Wood Interior Trim
This is a simple way to trim out the interior of your new vinyl or fiberglass windows. It gives the finished product a more substantial appearance and is preferable to the vinyl trim method. You will need a saw for this method and if you want to do a mitered joint, you will need a miter cut-off saw. A precision power saw will give the best results but the wood can be hand cut also.
First cut the wood trim to the dimensions of the inside of the window opening using whichever joint style you choose.
Always dry-fit the trim inside the window before caulking in place.
Next lay down a bead of caulk (make sure it's paintable) along the window sill and sides. After this cures it will support the wood trim at the perimeter so this bead should be at least even to the face of the window and preferably further out than the window face.
Put another bead along the face of the window where the wood trim will cover it. The bead of caulk on the face of the window should be very small since it is used for adhesive purposes only.
Fit wood trim to window pressing it firmly against the beads of caulk being careful not to press the perimeter of the trim deeper into the opening than the face of the window.
After the caulk has cured enough (usually overnight) apply a finish bead of caulk to the trim to fill any gaps between the trim and the window or opening.