Bondo for fixing exterior wood trim?

rosesr4meSeptember 14, 2007

My handyman repaired some wood exterior trim using Bondo rather than wood filler. I had never heard of this. What are the pros/cons of using Bondo rather than a good quality wood filler or epoxy. The house is in a southern (humid) climate.


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The Minwax 2 part wood filler is basically bondo, so I don't think there's an issue. Whatever he used, I hope the decay was completely removed and the remaining wood was treated with a hardener. If not the repair will fail.

    Bookmark   September 14, 2007 at 4:28PM
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The minwax filler uses the same chemical as a binder, but the filler is different.
It is much softer than bondo and easier to sand and work.

    Bookmark   September 15, 2007 at 11:12AM
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I think it was this past month's Old House Journal (I can't check, they're packed) that had a letter from a carpenter disputing a recommendation to use Bondo for wood patching - he said it doesn't expand and contract with the wood and he gets called in to fix it. (So I guess it makes him money!) I can't remember what he said to use instead, but the issue should still be on the newsstands. Or I can come back in a couple of weeks once I get my books and stuff unpacked.

Although I haven't used it outside, I used Quikwood epoxy filler for a furniture repair and found it extremely easy to work with - it's a doughy stuff that you just knead up and use, no measuring or mixing. Our painter used glazing putty for splits/cracks, nail holes, etc. in our exterior trim this spring.

    Bookmark   September 16, 2007 at 12:01PM
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Thank you all for responding. Sounds like I need to keep an eye on his repair since I know he did not use hardner on the remaining wood (though he did replace the damaged portion).

From now on, I will make sure he uses an actual wood filler.

Johnmari - Thanks for the Quikwood reference - I will check into that. Is it similiar to Minwax stainable wood filler (premixed)???

    Bookmark   September 18, 2007 at 6:15PM
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It's very little like the Minwax stainable wood filler; the Quikwood is a two-part epoxy product that must be mixed (by kneading). I wore disposable gloves working with it to avoid skin reactions like dermatitis and absorbing the chemicals through my skin. It is probably closer to Minwax High Performance Wood Filler than the latex-based water-soluble filler product. Personally I have not been that impressed with any of Minwax's products, but we're lucky enough to have other options where I live (SE NH) besides the big box stores.

    Bookmark   September 24, 2007 at 8:41PM
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I have read before that same issue..bondo does not shrink/expand same as wood and you will end up having to repair with a wood specific product. 'specially if you live in extreme weather area.There are ads for these products in This Old House magazine and the like.

    Bookmark   September 25, 2007 at 10:50PM
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Our painter used Bondo to repair old house windowsills that were falling apart due to old age/dry rot/moisture rot damage. We thought were were going to be out MANY dollars to repair or replace those antique windows. When he was done, you could not tell there had been damage. I never expected the windows to look that good. It seems very sturdy and looks great. Only time will tell how it holds up, but it's gone a year and looks perfect.

    Bookmark   May 26, 2011 at 11:08PM
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Many years ago when I owned a houseboat and lived on it, there was rotting wood framing in the stern sliding glass windows that made an exit to the back deck.

It would have cost a fortune to tear out the framing and rebuild it. An old salt told me to get some GIT ROT.

It is great, and saved me a lot of money. No tearing out the old rotten wood. Just get it dry. VERY DRY. Then drill or punch with an icepick into the soft wood to create channels for the compound to seep into. When it sets up, what you have is essentially epoxy-impregnated wood that looks like wood, can be painted even, and is now permanently fixed. I lived on that house boat for 10 years and never had a leak around the window again.

So I recommend GIT ROT. I saw it at a boat supply store a year or so ago. I do not work for them.

    Bookmark   May 27, 2011 at 1:34AM
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my experience is that old house people are rather fond of bondo. i think it was the right tool for the job.

    Bookmark   May 27, 2011 at 7:58AM
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Bondo and Minwax High Performance filler both use styrene monomer as the hardening agent with benzoyl peroxide as the catalyst to start the reaction.

The fillers are not the same however.

Bondo uses Talc and Magnesium Carbonate, while Minwax Filler uses Talc and Calcium Carbonate.

Magnesium Carbonate has a Mho's hardness of 3.5 to 4, while calcium carbonate is slightly lower at 3.

The styrene monomer after reaction is going to drive the hardness of the material also.

Having used both materials, Minwax is a lot easier to sand than Bondo.

I have replaced Bondo repairs that showed separation from the surrounding wood, and have seen other Bondo repairs nearby in the same wood that had produced cracks in the latex paint film.

I use Bondo for cars and metal and Minwax for wood.

If you catch the Minwax as it hardens you can use wood chisels to do preliminary shaping of repairs, then wait till hardening in complete to finish shaping and use sandpaper to blend the repair in.

Under a coat of primer and paint the repairs are not detectable.

    Bookmark   May 27, 2011 at 11:04AM
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Bondo-style (polyester resins, fillers, etc) indoors only. Outdoors it sometimes will last a few years, all too often fails in two years. Two part epoxy systems that include a first step to penetrate and solidify and then a filler step have a much better track record for me. Abatron and Advanced Restoration Technology are two such systems. A.R.T has a gel epoxy that is very easy to use and extremely flexible. It has a higher unit cost than Abatron's, but you can apply the consolidant and the filler the same day; with Abatron it's strongly recommended (and I vouch for this) to allow the consolidant to cure. I also lean toward A.R.T because I have used so much Abatron I've developed a sensitivity to it.

    Bookmark   May 27, 2011 at 7:17PM
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About five years ago, I restored three rotted window sills on a 175 year old house using several two-part epoxy products sold by a company called Rot Doctor. The products are similar to the Abatron system mentioned above. I used a liquid sealer and two fillers -- a gel that seeped into deep spaces and a paste filler for shallower spaces. After everything dried and cured, I was able to sand, prime and paint the window sills. The products are expensive and nasty to use, but the end result is fantastic. The sills are like concrete and look great after all this time.

Here is a link that might be useful: wood restoration products

    Bookmark   June 2, 2011 at 3:03PM
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I've seen this repair approach many times...and it fails every time. I agree with the writer who mentioned expansion and contraction issues. The bondo will look smooth and factory-like initially, but depending on climate conditions a bondo repair will likely flake and pop off in less than a year. The correct (and long lasting) repair process would be to use construction adhesive and nails or screws to secure the new trim. Apply enough construction adhesive to allow oozing around the edges. Once dry, sand the construction adhesive smooth, prime, and paint. That will give you a long lasting, professional looking repair.

Here is a link that might be useful: Exterior Trim Repair Alexandria VA

    Bookmark   December 31, 2014 at 5:51PM
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Bondo, a 3M company, now markets a Bondo Wood Filler designed for interior and exterior use. I don't know how it differs from the traditional Bondo auto body product.

I have used the traditional Bondo product for exterior applications for many years. With proper attention to surface preparation and sealing after it has been applied, I have very good experience with the product. Some window sills are approaching the 15 year mark with no sign of flaking or any other sort of failure. I have also used it to fill rotted areas, after carefully digging out the rot and applying hardener, in the bases of porch columns. 10 years on they are fine. Maine certainly is a severe climate in terms of temperature variations, so I have to believe that proper preparation is the key to using this product. Doen't mean there's anything wrong with the others mentioned above, but cost savings in using Bondo are considerable.

    Bookmark   January 1, 2015 at 5:25AM
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All I know is that my attempts at using Minwax for exterior repairs were abysmal failures - didn't hardly last a year. After using Abatron epoxy system the repairs have held up beautifully for several years now - no flaking of paint whatsoever. Abatron may seem slightly more expensive than Minwax, but not when you consider longevity of the repair.

    Bookmark   January 2, 2015 at 11:14AM
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Interesting information. I repaired a bunch of sections of my wood porch with the Minwax two-part filler. It will freeze, thaw, get wet, dry out, take a hard western sun, and on top of all of that it will be walked on. We'll see how it holds up!

    Bookmark   January 5, 2015 at 9:08AM
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I've tried various fillers over the years both on cars and houses and they have almost all failed. I did have some success (>3 years) with an epoxy system that involved first saturating the wood with a very thin liquid epoxy and then applying a thicker paste to fill the gaps. The product I used seemed very expensive, but did hold up at least as long as I was there.

Replacing the rotten parts and solving the water problem that led to the rot is the solution that has worked best. I've had much more success in making a "dutchman" to replace rotten wood when replacement of the whole part was impractical for some reason. The wood containing the rot is removed with a saw or router with clear margins (like excising a tumor, you want to take it well back into solid wood so the fungal mycelia are not left in the repair area) and new wood is filled in using a suitable water-resistant glue. The repair is shaped to match the old work and lasts a long time assuming the source of water is addressed. Similarly when repairing a rusted car panel it is far better to bite the bullet, cut out the rusted section and weld in new steel than to use bondo, lead, library paste, or wood filler to fill the holes. Replacing like with like is the most likely route to a successful long term repair.

    Bookmark   January 5, 2015 at 4:35PM
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I used Bondo on exterior door and window trim, one north, one south, about 10+ years ago - before I knew different. I also used wood hardener after digging out the rot and before the epoxy. I'm ashamed to admit the south repair hasn't even been painted. It's all holding up well.

    Bookmark   January 8, 2015 at 4:47PM
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The best wood filler on the market is an elastomeric epoxy. It expands and contracts with the wood, any other methods such as bondo, wood filler, even minwax two part, do not. If you want your repairs to last, do not use anything else.

    Bookmark   January 25, 2015 at 11:06PM
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