Repair Wood Rot: Epoxy Wood Filler

How to fix rotten wood with epoxy wood filler. I'm going to show my repair to the bottom of my exterior door trim, but you can also repair other wood rot, like window sills and frames, porch columns, decorative wood, and so on. These are cosmetic fixes and not "structural fixes" (repair to sill beams for example and other load bearing structures).

After scraping old paint off our exterior door, I noticed something that wasn't good. Some of the trim around the bottom of the door was rotted out. So I pried off the middle board shown above, and found that matters were even worse than I thought. The sill beam below the door had rotted as well! Oh boy...

Here's what the sill beam looked like with most of the worst rot removed. But you can see that in order to repair the sill beam (the 6"x6" beam on top of the foundation shown above), I'll have to remove the left exterior door trim to get at the beam. Note: I don't have the sill beam repair article done yet, but I will soon.

What follows is only about making a cosmetic fix to the bottom section of the left door trim. That bottom section, although it doesn't look too bad in this photo, was very rotted (mostly on the back side) and very soft up a few inches from the bottom. This is what happens when wood is in contact moist surfaces (the steps, during rain and snow). Also the wood grain at the end of the boards is particularly good at sucking up moisture.
Note: You can click on any photo to enlarge them.

Rather than remove the whole piece of left door trim, it seemed easier in this case just to cut the lower few inches off with a sawzall, make the rot repair with wood epoxy, and then reattach it later. I could have manufactured a new matching piece of wood, but it would have been quite a bit of work, as the piece was unique in size and notched out in the back, and also was not an available trim piece from home stores. I remembered seeing an article online about repairing rot with epoxy, so it was off to Google to research "rotten wood repair", as I was also eager to learn about rot repair for the fun of it as well!

After researching it was time to repair the rot. Here's what I did, using 2 products:
1. Liquid wood hardener (resin that penetrates soft wood, making it hard again)
2. Epoxy Wood Filler (2 part epoxy to rebuild/replace the rotted wood)

Here's the bottom of the door trim. You can see the rot from the front doesn't look too bad, but much of it was so brittle it just came loose in my hands, after this photo was taken.

The wood rot was worse on the back. The rotted wood was completely dry, but very soft and crumbly. Note: I'm in these photos as I was using the laptop as my camera here, and had to see what I was shooting. Please excuse my large nose!

Step 1: Remove Unstable Rotten Wood

The first step to repair wood rot with epoxy wood filler, is to remove any rotted wood that is loose, crumbly or very soft. You want to get down to reasonably stable wood. It can be a bit soft though. I used a screw driver, like a dentist removing tooth decay. Wire brushes work well for other flatter surfaces. The wood should also be completely dry, or as dry as possible before going further. This might mean using a heater, or fan, or whatever to dry the piece out. Or if it's an exterior piece you might need to cover the area with a trap for a while to thoroughly dry it out. Less than 20% moisture content is the recommended level, but who has a moisture meter? Just use common sense and get it as dry as possible.

Step 2: Drill Holes in and Around the Affected Area

Drilling holes helps the liquid wood hardener penetrate deeper into the wood. I should have drilled hole a bit bigger. 1/4" is recommended. Don't drill all the way through!

Step 3: Apply Liquid Wood Hardener

There are a number of brands out there. I just happened to have quick access to this one (Minwax, High Performance, Wood Hardener). This can was about $9.50 at Home Depot.

Lather on a few quick coats with a cheap disposable bristle brush. Be sure to get every nook and cranny. Then let it dry for 2-3 hours. When dry, the resin that has penetrated the soft wood will make it hard and stable again. My glove here looks weird as the lower half ripped off, so it's not some special fingers only glove, ha!

Step 4: Add Screws for Strength as an Extra Measure.

The screws are optional, but are not a bad idea, as they act like rebar (reinforcing bars used in concrete). The screws give the epoxy wood filler something to grab on to and will add strength to the repair. I used exterior deck screws.

Step 5: Mix up a Batch of Epoxy Wood Filler

This just happens to be Bondo epoxy wood filler. Many people have used regular Bondo for fixing wood rot before with no issues, but a quick Google search will also turn up just as many people saying that the traditional Bondo (used for auto body repair) isn't a good idea to use on wood rot repair, as it does not expand and contract with temperature changes and that a dedicated epoxy wood filler, made for wood rot does. The product above is from Bondo and say it's a "wood filler". I've never seen it before (might be the same as the auto Bondo, who knows). Anyway the photo on the front looked convincing so I figured I give it a go, as other epoxies I found online were more expensive. I'll give follow up, updates as to how it holds up in our harsh New England winters and humid summers.

First you fill the supplied cup with the wood filler. Forgot to put my gloves on here.

Then you squeeze out a bead of epoxy hardener to a length of the diameter of the cup (see your product for specific directions).

Mix the Bondo and the hardener up well with a disposable plastic putty knife.

Step 6: Apply the Epoxy Wood Filler to the Rotted Wood

Be sure to goop it into the nooks and crannies. I used plastic putty knife and screw driver to work it into the pockets of the rotted wood. Bondo suggest that you build up layers no greater than 1/2 inch thick at a time.

Bondo sets up quickly, so you'll be mixing up a few batches and applying it in layers if you have to build up a large section as I did. I'm sure there's a neater way to do this (you can build a form out of plastic), and I'm sure some people are better skilled with their putty knifes (you can wet your putty knife with lacquer thinner to keep the wood filler from sticking to it). I didn't bother much with neatness and it shows, but it didn't seem to matter. All this extra bulk will be sanded away. Let it cure for 30 minutes. It'll get warm, as the chemicals do their thing.

After about 15 minutes with a 5" orbital sander, I was able to craft a very close replica of the original trim piece, complete with notched cut out on the back side (the house siding goes behind that notch). No more wood rot here. And once primed, painted and reattached, this piece will be rot resistant, as it's epoxy, not wood.

More photos and story coming shortly of the piece being reinstalled.--Peter

Minwax, High Performance, Wood Hardener
Bondo Home Solutions Wood Filler (No info. on 3M's website, they make the stuff)
Popular alternative: Abatron LiquidWood, and Abatron WoodEpox
Another alternative: Conserv Epoxy
Another alternative: PC Products
Wire brush, screw driver or whatever to remove the rot
Disposable bristle brush
Plastic putty knife
Shaping tools: Rasp, sandpaper, sander, etc.

The Family Handyman: How to Repair Wood Rot with Epoxy
This Old House: How to Repair Rotted Trim with Epoxy Wood rot repair information and products
PC Products: Product video

My Standard Disclaimer: This is what I did. This does not mean this is the best way, the right way, to building code, or even safe for your needs. So you are on your own with your project. I make no promises about the information presented here. I'm just a do-it-yourselfer, not a professional at all, sharing my story. So if something goes wrong with your project, you are on your own. Good luck, and have fun!

All content and photos, copyright 2009, Dover Projects.
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  1. Did you know that you said the word wood 50 times in this article. I am so childish - isn't it great?
    Seriously - great write up. I don't do home repair, but I think I can with your instructions.

  2. I'm afraid you'll find you needed to use the real and expensive wood filler, not this.

  3. Hi Anonymous. Thanks for your comment. Do you think this is the same stuff as the auto bondo, only with a different color and packaging? Have you had experience with this Bondo brand wood filler? Thanks very much for a bit more detail to help out all the readers. Thanks!--Peter

  4. I'm very impress! I knew there is no way I can stop by HD and buy a wood that will fit under the window still. So with some work I can "make" a piece that fit from the original!

    will give it a try


  5. Looking, How did it turn out?--Peter

  6. Peter,

    This site is excellent - Thank you so much for all of your helpful articles. Being a new home owner, you make some pretty intimidating projects look do-able. Thanks again and keep up the great work!

  7. Wow! This is just a general kudos for this blog. I am so GLAD a friend forwarded this link to me. Great stuff here.

  8. Thanks very much for the nice comments above. I have many more projects that I've photographed but have not had time to write them up. Must get to them!--Peter

  9. I love your blog. Started with this entry and got valuable info! Will bookmark and continue to follow. Thanks for your experiences!

  10. NEW!!! DOVER PROJECTS ON FACEBOOK! Join in with your DIY spirit! Click on the BIG blue box for "Facebook, Dover Projects" in the right column above! See you there!--Peter

  11. To effectively get the moisture out of rotten wood, remove all the crumbly stuff then treat the area with auto antifreeze solution. Brush it on liberally, let dry & reapply. The solution has a tremendous affinity for water & will go to any moisture in the wood. It works by killing spores & any organism that contributed to the wood rot.
    When completely dry, treat area with Minwax Wood Hardener & then use the Bondo product.
    If you don't kill all the spores etc, the wood will continue to rot under the filler mat'l

  12. would the same procedure work for splintered wood? I bought a desk and a piece came splintered--cannot take back as I waited too long. It is about 2" high and 3" wide.

    Thanks Lynna

  13. Exactly the article I needed. I have seen epoxies used like this before, but I wasn't familiar with the hardener. The commenter who mentioned the antifreeze, that was helpful as well. About to give this a go over the weekend on a rotted beam over a window. Thanks.

  14. Anonymous, sounds like it could be used to fix your desk.

  15. Shane, This grade of Bondo grade filler is not intended for load bearing structural parts like header beams over windows. But I also know you are very handy and can make a judgement call on using it to fix a partially rotted member.

  16. I am building a Sandbox Boat ( sandbox that looks like a boat ) and found your site , looking for some pointers . I would like to thank you for the tips on wood rot, I have the same problem at my back door and I was go to remove the whole thing. but now I am going to try the Bondo wood filler. Thanks for the tip.
    Tim from California

  17. Hey Tim! Would love to see pictures of the sandbox boat when you are done. For daily content, and a group of us sharing project photos and DIY projects, check out --Peter

  18. Thanks, just finished it yesterday , I am getting ready to go get the sand right now, I will try and send a pic when I get back.. Tim

  19. Well folks, it must be the season for epoxy wood fillers. I just finished filling weakened areas of a threshold. Contact points where concrete and wood met had disintegrated. The previous owner used a rubberized calk application trapping water and worsening the problem. One place on the side of the threshold was in bad shape, difficult to clean, and more difficult to fill. I used the hardener technique, drilling holes through the wood and soaking it good. Next time I run into this situation, I will try both antifreeze and hardener. It is getting colder here in the Southwest and there is a race to finish any kind of exterior painting because of humidity. I chose to use the 3M’s Bondo wood product as a cosmetic fix and deterrent to additional rot. After sanding and painting, I was able to calk successfully around the edge stopping water from seeping under the board. Replacing the threshold is inevitable but for now, it should not rot further (I hope) and looks great.

  20. Thanks for writing up your fix Andy!—Peter

  21. this is a great idea for a good fix. thanks for the information!

  22. You are so nice to put this on the internet for all of us novices as myself! I have to repair the bottom of my adorable wooden screen door and you instuctions are going to come in very handy! I hope it all works out and again thanks!!!!!!!!

  23. Anonymous. Should do the trick for you nicely! Come on over and join us on Facebook, if you do that kinda thing. Daily DIY fun and games!

  24. You made that wood look great! Nice replica!

  25. this was incredibly useful.. your a huge benefit to us all... it worked for a more cosmetic fix in my home, a silly window trim, but same deal... arigato-gozaimashita

  26. HI theres a thin strip of wood missing from our exterior doors. we want to put wood filler into it and sand it before we paint it. is an orbital sander the best type of sander for wood filler?

  27. once dried up, can the epoxy wood filler be screwed or nailed? Was thinking about using this to 'redo' the rotten area at the hinge area of a kitchen cabinet door. The hinge just gave way when i opened the cabinet door....

  28. I work for Abatron. It's a common misconception that the polyester products you used are the same as epoxy repair products. In fact, you titled your blog "Epoxy Wood Filler" but the Bondo you used is polyester. The solvents and thinners used in the polyesters evaporate during the hardening process, making the products shrink. I won't ramble on, but there is a real difference.

  29. Howdy, Peter? You're no Average Joe! You definitely know a lot of things about carpentry. I'm so lucky that I have the chance to browse your website. I'll definitely bookmark it interesting and educational house improvement tips. The method you have done from restoring that broken piece of wood was truly remarkable. By the way, I would suggest you use a mask and gloves for precautions. Have a nice day, my friend.

  30. I need to fill in some holes in an indoor bedroom this the way?


Please leave your questions or comments here. I love to get feedback. You can do so without an account or user names and passwords, etc. Just select "Anonymous" next to "comment as". Thanks!--Peter