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How To: Repair Weathered Window Sills

How to repair weathered window sillIf 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 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 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


damaged wood sill
Dig out damaged wood
Image Copyright: Scott Sidler

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.

liquid wood application
Apply LiquidWood consolidate
Image Copyright: Scott Sidler

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!



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8 thoughts on “How To: Repair Weathered Window Sills

  1. Thanks for the reminder that working with completely dry wood will a window repair job a lot easier. I’m considering to hire such a service for my shed because it’s starting to disintegrate at some parts due the simple materials used for its construction. Getting to fix its worn out window will ensure that there wouldn’t be a potential hole for animals to enter through.

  2. Hi, Scott – I’m contemplating the purchase of a 1905 home. It’s been covered with a layer of aluminum siding and then a layer of vinyl siding. If I purchase the home, I plan on stripping both the aluminum and vinyl siding. I know I need to be prepared for possible repairs to the wood siding, and I fully expect that the original wood window sills will have been cut off at the front to make flat for the siding. Any good resources for restoring the sills. I suspect that just adding the front back on somehow is a really bad idea and that the best route is to remove what’s left of the sill and install an entire new one….thoughts?

  3. Thank you Scott. I have used many of your repair procedures on our beloved home built in 1925..

  4. Superior advice and explanation. Nothing left to guessing. Great product recommendation. Looking forward to outstanding results. Off I go in search of product.

  5. How can I repair my outdoor cement cracked window ledge? It has also caused the area under the interior window cell to crack?? Thank you.

  6. I’ve experienced a new paint stripper that has worked well on removing 90 years of paint, “SmartStrip” by Dumond. What little scent it has is pleasant which is great because I’m using it in the house. Neutralizes and cleans up with water or denatured alcohol. Has anyone had more experience with this product? The last stripper I used was Jasco. Nasty. I’m a newbie so I have a lot to learn and this is the place that has been a great resource and brain saver. Thank you, Scott.

  7. Hi, I am trying to figure out what the measurement of the flashing underneath the cill of a dormer window on pitched victorian roof should be.

  8. Trick:

    Epoxies are fussy about ratios. Get it wrong by a few percent, and it’s weaker. Get it too far wrong and either it won’t set, or it will set up WAY too fast. (I’ve had fiberglass resin pots start to smoke and catch fire — similar chemistry)

    For mixing liquidwood, take a yogurt/cottage cheese container, and pour 1/4 cup of water in it. Mark the outside with a fine point permanent marker. Another quarter cup, another mark. Repeat.

    Now you can accurately measure the two components for liquid wood.

    For the solid stuff, get out your kitchen scale. Modern scales are easily good to the nearest gram.

    (Yes, you can weigh the liquids too, but do NOT spill on scale. The kitchen staff will not love you for that.)

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