(Or, How to Accurately Cut Parting Strips.)
Often when installing new parting strips on a restored double-hung window, we’ll leave the original header parting strip in place. This parting strip is usually packed with decades of grime, which we don’t clean as it can serve an important purpose in marking the cut line for a new parting strip. First, cut your 10˚ bevel on the bottom of the new parting strip and then over cut it by about 1/4″ on the top. Then, with the upper sash lowered completely, and the parting strip snugly inserted into its groove at the bottom, rub the top of the parting strip back and forth on the outside of the old header parting strip. The grime on the old wood will impart a precise mark that can be used as a guide when cutting the new parting strip. Cut along the grime line and you will have a perfectly fitting parting strip.