DIY Fixing Rotted Window Sills,

Rotten Wood  Repair With Epoxy



The sad truth about epoxy and wet rotten wood

How do you treat wood rot?

Step bystep DIY repair of a rotten window sill

Using anti-freeze to treat rot

Useful wood rot related links



Repairing Wood Rot - DIY Window Epoxy Repair
Wood Rot Repair with epoxy products

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We've been selling epoxy - marine epoxy (boat building - wood / fiberglass repair) industrial coatings - garage paint -

underwater epoxies - thick putties - tabletop resins and supplies since the early 1990's

There are lots of rot repair pages and epoxy products available. The attempt on this page is to put it all in perspective, outline the general basics and theories, and than (of course), mention the products we sell.


Despite claims by many companies, epoxy sealers, penetrating epoxies, etc. will not fix, damp/wet rotted wood. The epoxy may or may not bond to the wet wood (some epoxies bond to wet surfaces) but it will not 'drive the dampness out of the wood' and replace it with hard epoxy. At best the epoxy will stick the the wet rot surface, still leaving the 'water' seemingly 'attached' to the wood.

You will note that penetrating epoxy claims are always done with dry sawdust type rot, the type of surface where any liquid will soak into the material. The material in the epoxy that helps it soak into the fibers is generally a traditional solvent. Some products are actually upwards of 70% solvent and only 30% epoxy (which costs more than solvent). I believe 70% epoxy and 30% solvent is a better ratio.

I have tried several times to epoxy 'fix' spongy, soft rotten wood spots on my outdoor porch deck. I've tried all the tricks, and at best I get a hard crust over the area. Still not satisfied, I've always ended up ripping out the old planking and replacing it with new. And I've also found that the rot on the bottom side of the blank was much more extensive than the patch of soft rot on the top side of the plank and that that the 'penetrating' epoxy never got to more than about 5% of the problem.

Window sills, trim work etc. are a different story. They are small areas and no one is walking on them, so it doesn't really matter too much of the fix is not 100%. The goal often is not so much to completely remove the rot as it is to stabilize the surface so that it will coat a coat of paint. Also porch deck planks are easy to remove and easy to drop a new one in its place. Window sills and outdoor trim are a different matter. Replacing or matching them is often more time, money, or skill than most of us have, so that best we can usually do is just a 'decent' patch.

So, the steps below are the results of personal experience and feedback from customers. There are certainly no right or wrong ways but the method explained below, seems to a good one.


Optional (at this point) - find the rot and remove as much of it as you can.

rotten wood

Option 1) - some people treat the area with glycol (anti-freeze) or boric/acid-salts to kill the fungus spores. (NOTE: This is optional - the epoxy will kill the fungus spores to, but will not 'soak' in as well as the other 'fixes - including 'anti-freeze' see below. There is some concern about bonding to anti-freeze coated surfaces.

In personal tests, I found that anti-freeze penetrated the wood better than anything I have seen (it is attracted to moisture unlike other 'chemicals'). - try it yourself, put a piece of new yellow wood into some anti-freeze and watch the 'stain' migrate through the wood over the next few days. I also found that solvent thinned epoxies stick OK to anti-freeze coated wood and unthinned epoxies will stick decently enough too.

The method is to keep soaking the area with anti-freeze until it will accept no more and then let it 'dry' for as long as possible. This method does have a track record for stopping the rot.

For a third party article on anti-freeze (glycol) use with rotted wood try these links - ( article #1 (treating rotten wood) - (article #2 (more on treating rotted wood) -

Option 2) - treat the area with isopropyl alcohol. The alcohol is a solvent that will 'mix' with the water and evaporates much quicker than water. Thus the evaporating alcohol will carry with it some of the dampness/wetness in the wood. I suppose the alcohol might also kill some of the rot fungus.

Option 3) - cannot say I have tried it, but I suppose heat lamps etc. would help drive out the moisture and water in the rot area. The drier would would add in epoxy penetration. Also, if the wood were reheated just prior to applying the epoxy, the cooling down wood will 'draw in' the epoxy (as the air contracts and cools) deeper into the wood than if the wood were at room temperature. This 'apply to a cooling down surface' is not theory but a well established and proven technique in a number of epoxy application situations.

keywords= diy rot repair



find the rot and remove as much of it as you can (if you haven't already). Let the wood dry as much as possible.

See pre-treatments above (you can optionally let wood dry naturally, use anti freeze, use alcohol, use heat)


window sill rot repair with epoxy

Seal and reinforce the wood with a 'solvent thinned' thin epoxy. Our ESP 155 epoxy is solvent thinned already. Our Low V epoxy is solvent free - you optionally add 0 - 100% solvent to it. Keep repeating this step until no more epoxy will soak into the wood.


If the chipped out bad area is large, some people recommend carving a wooden plug to fill up most of the space - thus saving epoxy and/or epoxy putty and incorporating more 'real wood' into the repair.


rotten window sill repair

At this point you might fill the rot cavity with the chunk of wood and epoxy putty into place. Whether you use the wood plug or not, time to NEARLY FULL the cavity with epoxy putty. Use our Wet Dry 700 epoxy paste/putty or epoxy cream product. These epoxies will bond to damp surfaces (you can even apply it underwater!). You can also use our non solvent thinned Low V epoxy mentioned in Step 2 and our EZ Thick thickener to make an epoxy paste (approx. 2 parts thickener to 1 part epoxy).


how to epoxy rot

wood rot repair

Using an exterior putty (cheaper and easier to sand/smooth - buy from local hardware store) fill the remaining hole. Let harden and sand smooth. It is much easier to sand and shape the non epoxy putty.


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Repairing Wood Rot - DIY Window Epoxy Repair
Wood Rot Repair with epoxy products


wooden window repair epoxy

Using the same epoxy you used in step 2, coat the putty patch and the surrounding wood. This will 'seal' in the putty patch (locking it away from rain, ice and snow) and prime the patch and the window sill (or whatever) for painting.

You can add pigment to the epoxy, which will make it easier (few coats) to topcoat.


window sill repair rot

Paint the area with a good quality paint (100% acrylic latex or enamel).



"WOW! Just completed one coat of ESP 155, then Epoxy Cream to fair, two more coats of 155. I'm delighted with the finish. I just didn't order enough.
Thanks for great advice." --- M.B. 9/2010 (repair project)

Below is an article written by Dave Carnell - a chemist and scientist and something of an icon in the marine/boating world. I believe the article was written in 2000. Sadly, Dave died in 2011.

Chemotherapy for Rot

Once rot gets a toehold in wood it is difficult to cure completely -- it is like a cancer. Digging out the rotted wood will still leave spores and water in the sound wood. After you fill in the cavity with something like epoxy, the rot continues to flourish underneath. Products promoted to make rotted wood sound and stop rot penetrate only until they meet water, with which they do not mix. Under the solid repair rotting goes on. With one exception (more later), the commercial products sold to treat dry wood to prevent rot are completely ineffective against established rot in wet wood because they are dissolved in petroleum solvents and oil and water do not mix.

There are two commonly available inexpensive materials that will kill rot in wood and prevent its recurrence. First, there are borates (borax-boric acid mixtures) which have an established record in preventing rot in new wood and in killing rot organisms and wood-destroying insects in infested wood. Second, there is ethylene glycol, most readily available as auto antifreeze-coolant. Glycol is toxic to the whole spectrum of organisms from staphylococcus bacteria to mammals. All of the published material on its effectiveness against wood-destroying fungi and insects that I am aware of is the result of my investigations over the past 15 years.

Both borate solutions and glycol penetrate dry and wet wood well because they are water-soluble; in fact, penetration by glycol is especially helped by its extreme hygroscopicity -- its strong attraction for water. For both, the fact that they are water-soluble means they are not permanent solutions to rot in wood that is continually exposed to water-below the waterline and in ground-where they will eventually be extracted-dissolved out.

I first was interested in glycol as a wood-stabilizing agent, where it is in many ways superior to polyethylene glycol (PEG), and it was during this work that I realized the useful effect of glycol on organisms, though I was pretty dense in interpreting the first experiment.

The ladies immerse the stems of greenery such as magnolia branches in glycerin to keep them green. Glycol is very similar to glycerin in all its physical properties and much cheaper, so I stuck a magnolia branch in antifreeze. The next day it was brown. After the third attempt I tumbled to the fact that the glycol was killing the greenery. This was the reason that glycol never replaced glycerin in applications such as a humectant for tobacco and an ingredient of cosmetics and pharmaceutical ointments, though it had all the desirable physical properties.

I had two 2" thick slabs of a 14" diameter hickory tree that had just been cut. I treated one with antifreeze and left one untreated. I was looking at wood stabilization, not rot prevention. After about six months stored inside my shop the untreated control was not only cracked apart, but it was sporting a great fungal growth, while the treated slab was clean.

The local history museum wanted to exhibit two "turpentine trees", longleaf pines that had many years ago been gashed to harvest the sap that made everything from turpentine to pine tar. The trees delivered to us after cutting were infested with various beetles and had some fungal growth. I treated them with antifreeze outside under a plastic tarpaulin every few days for three weeks. They were then free of insects and fungus and have remained so after being moved inside and installed in an exhibit over four years ago.

I took three pieces from a rotting dock float that were covered with a heavy growth of fungus, lichens, etc. I treated one with antifreeze painted on with a brush, the second with a water solution containing 23% borates (as B2O3), and left the third untreated as a control. They were left exposed outdoors and were rained on the first night. By the next morning the growth on the antifreeze-treated piece was definitely browning and the borate-treated piece showed slight browning. After two months exposure to the weather the growth was dead on the antifreeze- and borate-treated pieces and flourishing on the control.

I have a simple flat-bottomed skiff built of plywood and white pine, which has little resistance to rot. After ten years some rot developed in one of the frames. It may have begun in the exposed end grain. It consumed the side frame, part of the bottom frame, and part of a seat brace fastened to the side frame. The plywood gusset joining the side frame to the bottom frame was not attacked. I excised the rotted wood, saturated all with ethylene glycol antifreeze to kill all the rot organisms, and there has been no further deterioration in four more years afloat with wet bilges. I have not replaced any pieces, as I am building another boat that can replace it; that is more fun, anyway.

I have a 60+-year old case of the fungus infection known as "athlete's foot". Many years ago it infected the toenails extensively. The whole thing was pretty grotesque. My dermatologist and druggist both assured me there is no known cure. About six years ago I started using antifreeze applied under the nails with a medicine dropper about every five days. The professionals are technically right. I have not completely cured it, but the nails have grown out pink and thinned almost to the ends and I never have any trouble with blistering, peeling, or itching between the toes as I had had for six decades. No drug company is going to have any interest in this because the information has been in the public domain for so long that there is no opportunity for any proprietary advantage. The various wood-rotting organisms cannot be anywhere near as tough.

There are two types of borate products commercially available for treating wood-solid sodium octaborate for making solutions in water (Tim-Bor® and Ship-Bor®) and a 40% solution of sodium octaborate in ethylene glycol (Boracare®). Their equivalents and more concentrated solutions can be easily prepared from borax, boric acid, and antifreeze at much lower cost. Keith Lawrence, editor of Boatbuilder offered to sell me advertising if I wanted to go in the business, but I might run afoul of patents (preparation for individual use is not prohibited), I would have to get EPA registration, and I could not deliver products anywhere near as cheaply as they can be made from raw materials available at your supermarket, drugstore, and discount store.

Glycol by itself has one big advantage over solutions of borates in either water or glycol. Glycol penetrates rapidly through all paint, varnish, and oil finishes (except epoxy and polyurethanes) without lifting or damaging those finishes in any way. You can treat all of the wood of your boat without removing any finish. The dyes in glycol antifreeze are so weak that they do not discolor even white woods. Once bare wood has been treated with glycol or the borate solutions and become dry to the touch it can be finished or glued. IN THE YEARS SINCE I FIRST WROTE THIS ARTICLE, MY EXPERIENCE HAS BEEN THAT GLYCOL BY ITSELF IS GENERALLY THE BEST TREATMENT FOR KILLING ROT. Gougeon's research has shown that borate solutions weaken epoxy joints in the treated wood. If a borate solution leaves white residues on the surface, it will have to be washed off with water and the surface allowed to dry.

If you decide you need to treat with both glycol and borates, this is my preferred process to treat rot. Once you find soft wood or other evidence of rot, soak it with antifreeze even if you cannot do anything else at the moment. Paint it on or spray it on with a coarse spray. Avoid fine mistlike spraying because it increases the likelihood that you will breathe in unhealthy amounts of glycol. Put it on surfaces well away from the really damaged wood, too. Use glycol lavishly on the suspect wood, which will readily absorb 10-20% of its weight of antifreeze.

Next dig out wood that is rotted enough to be weak. Add more glycol to wet the exposed wood thoroughly. Then add the 25% borate solution of the recipe below so long as it will soak in in no more than 2-3 hours. Then fill in the void with epoxy putty and/or a piece of sound treated wood as required. The reasons I use borates at all are: 1) it is a belt-and-suspenders approach to a virulent attack, and 2) over a long period glycol will evaporate from the wood; especially, in areas exposed directly to the sun and the high temperatures that result.

If there is any question about water extracting the glycol or the borates, you can retreat periodically with glycol on any surface, painted or bare, and with borate solutions on bare wood.

Glycol's toxicity to humans is low enough that it has to be deliberately ingested (about a half cup for a 150 lb. human); many millions of gallons are used annually with few precautions and without incident. It should not be left where children or pets can get at it, as smaller doses would harm them, and they may be attracted by its reported sweet taste that I have confirmed by accident. The lethal dose of borates is smaller than of glycol, but the bitter taste makes accidental consumption less likely.


Tim-Bor®: Solid sodium octaborate; dissolves in water to make approx. a 10% solution containing 6.6% borate (B2O3); about $3/lb. plus shipping.
Ship-Bor®: Same as Tim-Bor®; $19.95/lb. plus $2 shipping.

Bora-Care®: 40% solution of sodium octaborate in ethylene glycol; 27% borate content; $70/gal. plus shipping.

Home-Brew Water Solution of Borates:

All percentages for this recipe and the others here are percentages by weight. Based on U.S. Navy spec. of 60% borax-40% boric acid (this ratio gives the maximum solubility of borates in water); 65% water, 20 %borax, 15% boric acid; 15.8% borates; borax costs 54 cents/lb. (supermarket), boric acid costs about $4/lb. in drug stores (sometimes boric acid roach poison, 99% boric acid, is cheaper in discount stores); equiv. to Tim-Bor® or Ship-Bor® at 30 cents/lb. To make this solution mix the required quantities and heat until dissolved. The boric acid, in particular, dissolves slowly. This solution is stable (no crystals) overnight in a refrigerator (40°F.), so can be used at temperatures at least as low as 40°F.
Home-Brew Glycol Solution of Borates:

This is equivalent to Bora-Care® diluted with an equal volume of glycol to make it fluid enough to use handily; 50% glycol antifreeze, 28% borax, 22% boric acid. To make a stable solution you mix the ingredients and heat till boiling gently. Boil off water until a candy thermometer shows 260°F. This removes most of the water of crystallization in the borax. This solution is stable at 40°F and has a borate content of 26%. With antifreeze at $6/gal. and borax and boric acid prices as above, this is equivalent to Bora-Care® at about $15/gal.

Links to mentioned products

Products offered by Progressive Epoxy Polymers, Inc.

LOW V™ (thin, solvent free epoxy - add solvent to this to make a penetrating or priming epoxy. Add thickeners to form a paste) - Click Here (low V epoxy)

ESP 155™ (our own clear primer epoxy with approx. 25% solvent - add more solvent for an easy penetrating epoxy) - Click Here (epoxy sealer)

WET DRY 700™ (epoxy paste - bonds to wet surfaces) - Click Here (underwater epoxy)

EPOXY CREAM™ (epoxy paste - bonds to wet surfaces) - Click Here (easy to use epoxy filler putty)

EZ THICK - (thickening powder for epoxies) - Click Hee (epoxy thickeners)




Penetrating epoxies -

Treating rot (this page!) - www.epoxyproducts.com/rot.html

Competitor's penetrating epoxy solvent list - www.epoxyproducts.com/solvents.html

More on epoxy primers - www.epoxyproducts.com/primer.html

Sealing wood (tests) - www.epoxyproducts.com/woodseal.html




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Repairing Wood Rot - DIY Window Epoxy Repair
Wood Rot Repair with epoxy products