Windowsills, Sunlight and Rain

Its that time of year again – finally.

With temperatures steadily climbing, we can finally begin opening our windows and letting in the fresh spring breezes – but with those breezes come spring showers and additional challenges for the clear finish on our interior windowsills.

The finish on your windowsills has a complicated task: it has to look like a clear interior finish, yet it also has to repel water (often for hours at a time) and it has to resist the destructive ultraviolet rays of the sun. Chances are your windowsills were finished with the same clear protective coating found on the inside of your windows and on your interior trim and doors.

That finish is probably an oil-based interior varnish,various versions of which have been used inside homes since the 1920s.

While it is fine for wood that does not have to withstand hours of either sunlight or standing water, interior varnishes soon begin to break down, fade away and show water spots on windowsills. What might now just seem a bit unsightly may soon be permanently marred with black stains when the water seeps through the weakened finish and into the wood.

To prevent that from happening, take the following steps:

1.) Sand the old finish lightly with #180-grit sandpaper to remove water spots and to etch the finish;

2.) Wipe or vacuum off the fine sanding dust;

3.) Apply a coat of a clear, oil-based interior-exterior finish in a satin sheen.

Exterior finishes contain ultra-violet reflectors and are slightly thicker than interior finishes. If you select a satin sheen and just apply one coat, it will come very close to matching your existing interior finish. Also, select a brand that is labeled “Interior/Exterior” (such as Minwax Helmsman Spar Urethane) as these are not quite as thick as Exterior Only finishes (sometimes also called Marine Varnish).

The additional coat of an interior-exterior finish over your existing finish will strengthen it without changing its appearance drastically, but to make sure the two finishes adhere to each other, don’t skip the light sanding first!

Good Luck!

-bj