Polyester Versus Epoxy Wood Fillers

Opinions vary on the pros and cons of epoxy and polyester-based wood fillers. May 24, 2010

I'm looking for support on an issue Iím dealing with. My manager believes Bondo to be the best filler for defects in wood (natural and man made). I disagree, feeling that epoxy and the appropriate filler to be far superior. West System G5 and 410 filler is my choice due to better adhesion and stability/strength. We spray vinyl washcoat and color with catalyzed lacquer. Are there any problems with finish adhesion with either product? Can anyone out there point me to some solid documentation on this?

Forum Responses
(Adhesive Forum)
From contributor F:
Bondo is an epoxy with fillers so, in a way, you're both right. I'd suggest using Famowood wood filler as an alternative. It's considerably less expensive and performs quite well.

From contributor Z:
Bondo and most body fillers are made of polyester resins. Not the same as epoxy. To me, both Bondo and epoxy are not suitable for regular furniture and cabinets. Too much fiddling. What's wrong with latex wood fillers in a tube? What are you fixing?

From contributor F:
Contributor Z, you're right. I just looked at their site and Bondo is the trade name for a wide range of products. I also agree that neither Bondo nor epoxy are ideal fillers for wood.

From contributor A:
Bondo is a trade name for a common automotive filler that is polyester resin and typically talc. The Bondo name is used like Kleenex to refer to any mix of polyester filler. Epoxy is not polyester. Both are excellent for use as filling agents with wood. Epoxy is an excellent glue, and when mixed with the appropriate filler materials like wood dust or microspheres is a good filler for wood. Good polyester fillers are made by Bondo (available at pro autobody stores), NAPA, 3M, and Evercoat (marine). They use glass microspheres as the filling agent versus talc (typ. Bondo). At $12 a gallon it's hard to beat NAPA's MicroLight. Minwax sells their version for big bucks and is a waste of money. Bondo is also known as architectural filler in the woodworking industry. A couple of things to consider when using Bondo.

1. Bondo will not stick to paint.
2. Do to density differences and absorption it helps to spot prime with BIN shellac primer when topcoating.
3. If it sticks to steel it will stick to wood.

Polyester resin sticks well enough to wood to be used as a filler. It does not adhere as well as epoxy. It is just as stable and fillers in wood do not have to be strong. We typically use three fillers in the shop.

1. Pinholes-Muralo brand Spackle.
2. Nail holes-Zar waterbased woodfiller (much better than Elmerís).
3. Screwholes, big gouges, missing wood, etc.-NAPA or 3M polyester filler.

From contributor M:
I have the exact same disagreement with my employer, just in the opposite direction. The shop foreman (who has absolutely no financial accountability in matters) will always buy the most expensive wood epoxy he can find. The stuff he gets stinks much worse than Bond-o, takes a lot longer to harden, and doesn't sand quite as nicely. Yet, that's what you're supposed to use. I have yet to see what rule book he's been reading, but I do it because that's what you're supposed to use.

I've personally used Bondo for years on paint-grade items. Works just fine, bonds just fine, you just have to mix it right (which isn't hard). I prefer a more hot mix since it cures sandable more quickly.

As far as whether epoxy is superior or not, the question would be, what advantage does epoxy have over Bondo? I'm sure it does have an advantage, but I have never had a problem with Bondo. It's less expensive than the epoxies I've seen, easy to use, has a bit of a noxious smell, but is just plain effective. Just because it was made for cars doesn't mean anything, it's merely polyester resin with a filler added to it. Polyester bonds to all sorts of stuff, and also clear polyester coatings are great wood coatings if you need high build and durability - just as an example of using polyester with wood.

From contributor J:
There is always a need to fill some big spots in woodwork when it's paint grade or covered. I only use Bondo when it will be weather exposed. The quicker, easier, and cheaper choice for me is good old Durhamís Rock Hard Putty. Water mix and cleanup, cures quick, expands as it cures, and easy to work when hard. No adhesion problems either.

From contributor C:
I am not a professional carpenter but I was doing a restoration of some circa 1950ís wood picture window frames that were rotted out at the base of each vertical section of the frames. I gauged out the rot and it ran in a couple cases right into the house. I initially tried with my limited experience to use standard wood filler but sadly it never cured and crumbled when I tried to sand, so I tore it all out again and cleaned it up. Some spots had to be filled by up to 5-6 inches of filler it was so rotted. I discovered what people were saying about using Bondo automotive body filler and so I tried it out. I have to say I was immediately amazed at how awesome this stuff was. It took a couple of layers to get the hang of mixing it and applying it within the 3-4 minute cure period but once you get the hang of it you can fill almost anything and be done with it in minutes. Not only that but I found it really easy to shape after it cured for a few minutes. I just used a scraper and scraped away the excess and so if you time it right you barely need to sand it afterward. Just don't wait too long or you'll need a chisel and file to sand it away. Anyway, Bondo is the stuff for big jobs, can't beat it.

From contributor M:
When I mix up Bondo I also go through the whole job and goop it into any nail hole, screw hole, crack, open joint, any surface defect (obviously paint grade only). Itís just plain hard to beat the stuff. I actually use Bond-O to make fishing sinker molds (pyramid sinkers). I use a store-bought sinker, spray with WD40, and dip it into the Bondo until the stuff hardens, pop out the sinker and you have the perfect heat-resistant lead sinker mold.