By far the most serious possibility, however, is that eventually a sash cord will break and need replacing. When this happens you should replace all four, because if one goes the rest are sure to be near breaking-point. In any case the hardest part of the job is dismantling everything; fitting four new cords is very little more trouble than fitting one.
Begin by levering off the staff bead. Swing the lower sash inwards, tie a length of string to each sash cord just above the sash, and cut each cord below the knot, letting the weights down gradually to the bottom of their compartments. With the string now attached to the cord, there’s no danger of being unable to retrieve the weights from their pockets. Lift out the lower sash, remove the parting bead, pull down the upper sash and repeat the process. Lift that out too, and remove the old cord and fixing nails from each sash.
Now that you’ve got the sashes out, you can take the opportunity to do major surgery on them if necessary. Where a corner joint is rickety, dowels make a neater and more professional repair than a metal plate. You’ll need to cramp the sash firmly in place on a workbench first, both to hold it steady and to keep the joint tight while you drill the dowel holes. The joint will almost certainly be a mortise and tendon, so the best place to run strengthening dowels is sideways through the tendon, or perhaps lengthwise on either side of it.
If one of the sash members is cracked or rotten, it may be possible to remove the bad piece by sawing lengthwise, and to replace it with new timber — cut slightly too large, glued, nailed and finally planed off flush. For both these jobs, use urea-formaldehyde adhesive, which resists damp.
Sometimes a sash sticks in its channel because it has warped or swollen. In this case, removing a few shavings from the offending part with a plane may be the answer.
Use a blowlamp or chemical paint stripper to remove layers of old paint if it’s in bad condition. and then coat the timber with wood primer. Alternatively just sand the old paint- work down and spot-prime any bare patches. Then apply the rest of your paint system in the usual way.