How To: Repair Weathered Window Sills

If you’ve got badly weathered window sills, you’re not alone. Wood that has been exposed to the sun’s destructive rays for years without a protective layer of paint can begin to weather so badly that it may appear to be beyond saving. But with the magic of a good wood epoxy you can save damaged wood like this and make it look like new.

 

For epoxy repairs, there are a lot of products on the market but the ones I prefer are Abatron WoodEpox and LiquidWood. They are easy to work with, don’t require any complicated mixing or portions to screw up and have zero VOCs. That’s why they are one of my sponsors, they make a fantastic product!

LiquidWood

LiquidWood is an epoxy consolidant which essentially means it is a low viscosity liquid (kind of like syrup) that is combined 1:1 (meaning you mix half part A and half part B). Its thin consistency allows it to penetrate deep into the wood and strengthen rotted wood back to where it should be.

When used in conjunction with WoodEpox it also acts as a primer (or glue) to help hold the WoodEpox patch in place.

WoodEpox

WoodEpox is a two-part system like LiquidWood except that it is a paste filler instead of a liquid. Parts A & B are mixed together (in equal portions again) and they can be molded, troweled, filled into any void or portion of wood you need repaired. Once mixed together the paste has a play-doh like consistency that is easy to work with.

WoodEpox can also be thinned to whatever viscosity you need. By adding some LiquidWood or Acetone you can get a paste filler that can be poured into fine cracks and deeper holes.

 

How To Repair Weathered Wood Sills

 

Step 1 Clean the Surface

Remove any existing paint and dirt first from the area by scraping and wiping the area down with a rag moistened with TSP.

Step 2 Remove Loose Wood

Any wood that is loose or rattling around won’t be much good for your sills. Dig it out with an awl or 5-in-1. Anything that is still attached, even if it is a little soft, should be just fine to leave in place. Vacuum up any remaining loose debris.

Step 3 Dry Things Out

Epoxies work best on dry wood. Dry wood sometimes feels like a myth down here in Florida, but your wood should have a moisture content no higher than 12-15%, depending on your climate. Arid climates may have a moisture content below 10%, whereas humid climates like Florida will probably read below 14% for stable wood.

Also, avoid doing these repairs when rain is expected. If it does start to rain or you expect dew in the mornings, cover the area to repair with plastic sheeting to keep it dry.

Step 4 Prep the Area

Put down drop cloths and be sure to wear gloves and safety glasses when working with epoxies. These are chemicals that are not designed for skin contact and they will bond to almost anything if you drop or spill them. Keep some Acetone handy for spills and clean up.

Step 5 Apply Consolidant

In a disposable container, mix up your batch of LiquidWood (remember the 1 to 1 proportions) and let it setup for about 15 minutes. Once it is ready, you can apply it to the surfaces that require stabilization and repair. I prefer using a glue brush to help the epoxy work down into cracks and crevices.

Apply it liberally. The wood will likely drink it up, so a couple of applications over the course of a few minutes may be necessary.

Step 6 Apply Paste Filler

Give the LiquidWood a few minutes to setup and start to get tacky. Hot humid weather will speed up the process and cold days will slow it down. Once it’s ready mix up, your WoodEpox.

Mix equal amounts of both Part A and Part B by kneading them in your hands until you have a uniform color.

Next press the paste filler in place to fill the void. Be generous with your proportions. In my opinion, it is better to have too much paste filler and then sand it down later than to have to do another patch because you left the patch just shy of the surface.

Really press it into place to fill the whole void and then smooth the surface with a plastic putty knife.

Step 7 Sand Smooth

After the epoxy has had time to cure, you can come back and sand the surface smooth. Larger patches will cure faster than small patches. Chisel it, plane it, sand it, carve it, do whatever you need to get the surface to the shape and level you desire.

WoodEpox sands very easily, so it won’t take much effort or a heavy grit.

That’s it! Once the sanding is done, you’re ready to prime and paint your repairs. Once you put that finish coat of paint on, no one will ever know there was a repair.

I’ve rebuilt entire sections of a window with just paste filler and they have performed just like the old-growth wood. Follow these instructions and you’ll be just fine.

 

Final Tips

Epoxy repairs can be pushed out of the wood by moisture in the wood if you’re not careful. The way to prevent this is to avoid skim coating window sills or other wood. Just fill the checks, cracks, and voids, but don’t cover the surface of the wood.

John Leeke covers the topic in depth in his book Save America’s Windows which I recommend reading whole-heartedly.

One last thing, don’t use Bondo or some other auto body filler for exterior wood repairs. It was never designed for this kind of work and will fail rather spectacularly. But that’s a topic for another post!