I am replacing a bathroom window with one that is slightly bigger than the original. I will need to trim the length of the existing wood siding a few inches to match the trim of the rest of the house but would like to do it in place. I am worried about damaging the substrate if I use my skill saw. Does anyone have a method for doing this safely?
Set the depth of the blade on your circular saw to be just deep enough to cut through the siding. Err on the side of slightly too shallow, and come back with a chisel, utility knife, or handsaw to finish.
If your circular saw's base plate is not long enough to sit across two planks of siding, you could fashion a jig to hold it. The jig could have a long base so it sat across 3 or 4 planks of siding. The jig just needs to be a long piece of plywood with simple stops on the 4 sides; the stops would hold the saw's base plate in place.