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Check around the new garden window for any gaps or empty spaces that need trim. If the window you chose is slightly oversized for the opening, you may want to trim out the area left over.
If you need to do any filling on the inside around the window, take measurements and cut boards to fit. Make sure that the trim pieces you use match the trim around the interior of the window.
Trim for the interior of the window can be purchased at a home improvement store. 1"x3" stock is used in this project. You'll want to make sure the thickness of the trim does not extend beyond the overhang of the bottom shelf of the garden window.
Measure the lengths of trim and cut to size with a circular saw or table saw. For a finished look, miter 45-degree corners using a miter saw.
Before installing the trim, you'll want to prime the pieces. Use a high-quality primer, which has more pigment than less expensive primers; it may allow you to get away with only one coat of paint.
After priming, set the trim aside to dry completely.
To install the trim, put a bead of sealant down the backs of the pieces and set them into place. Check for plumb and level. Hammer the trim into position with finish nails.It doesn't matter where you start installing the trim; just make sure it fits snugly before you nail or glue it into place.
Using paintable latex caulk, caulk the seams around all the trim (Image 1), and fill all holes and cracks with caulk.
After installing and caulking the trim, paint it with a semi-gloss paint made for kitchen or bath applications. Such paints hold up better in moist environments like garden windows.