Auto-Body Filler as Wood Filler

Tips on how to work with auto-body fiberglass and resin filler products. December 30, 2007

I was wondering what luck you have had with Bondo as wood filler? Will it show up over time? I will be filling screw holes in solid poplar wood.

Forum Responses
(Finishing Forum)
From contributor P:
I use it all the time for painted jobs. I haven't seen it shrink or come loose yet.

From contributor J:
It was a standard material in the last pattern shop I worked at. Actually, we used a generic lightweight body filler because it was cheaper, and we used it in large volumes (imagine a gallon of filler, a whole tube of hardener, and a jiffy mixer).

It's harder than fillers explicitly formulated for wood, and this can make sanding it flush with the surface a bigger pain than it needs to be. Sanding it before it's fully hardened destroys sandpaper and often tears out the filler. Neither does the hardened stuff slice easily with a chisel. I think softer stuff might pay for itself in reduced labor, if you're just filling over screws and doing it frequently.

From contributor E:
Contributor J hit it dead on. The one thing I'll add is, whenever I use Bondo for something larger than a nail hole, I work it before it completely hardens. Put on just enough to fill the defect, let it set up long enough to get firm (couple of minutes depending on how you mix), then flush it with a nice sharp chisel. Work it too soon and you'll have a mess, too late and it's hard as a rock. Sometimes it's just easier to plug over the screws, especially in solid wood.

From contributor K:
I've only had it come loose if the underlying material was bad or if I mixed it wrong or tried to work it too long. I smooth it when it gets to the "hard soap" stage and use a shurform file for the initial work. I tend to overfill in hopes that I get any bubbles out. Work fast!

Keep both grey and pink hardener on hand. The pink works well for mahogany, cherry, etc. You can also tint it using epoxy tints (Tap Plastic) and maybe TransTints.

From contributor M:
Always keep a gallon in stock. Never had a problem with it coming loose. I like it because it sands easily.

From contributor T:
I have also used Bondo lightweight, mostly in shop. Great cabinet in a can. I also use quite a bit of epoxy sticks (several different colors) out on job sites. Haven't had any problems with them either. If it's part of a profile, I'll wait until it's just about set up, then detail it with a double edge razor blade. As with any of these products, you have to be fairly quick. Once it's set, sanding can be a real issue.

From contributor A:
The best stuff is found at auto body shops or pro grade parts stores. NAPA carries a product called Microlight ($12 a gallon). Best polyester filler available. The Bondo brand stuff is expensive and doesn't really sand that great. 3M makes a pinhole free product that is super high quality, but is $24 a gallon.

The reason it gums up sandpaper is the resin will not cure in contact with air. That first layer is not cured, hence it gums up paper like crazy. Most people leave a little lump and cut the top off with a chisel. The remaining putty is fully cured and hard as rock, but sands like butter. You will rarely get print through a good coating of primer.

From the original questioner:
Thanks for the responses! Just wondering if anyone has gone back to see if the Bondo projects through the paint a few months later. My concern is that the wood moves and the Bondo does not. The wood would be primed right away and painted a week or so later on site.

Also, can you purchase the Microlight filler online? Is it a West Systems product? I have run a few searches and it seems to come in a 4.2 oz can and a 4lb can. Thanks again for the responses!

From contributor A:
They happen to have the same name. One is a Bondo replacement. The other is an additive by West Systems for epoxy resins only. I tried to find it on the NAPA website, but it would have taken forever, and you have to purchase it at the store. If you don't have a NAPA near you, I would look for a pinhole free version like the 3M. Bondo sells many versions, but the most common they sell at Autopalace, etc. isn't very good. At the end of the day, any of these products work great for filling larger nail/screw holes. I thought it was worth mentioning the better versions.

From contributor M:
We use it to fill screw heads when laminating P/Lam. Keeps the trimmer from going in screw hole when you least expect it.

From contributor U:
At your local NAPA auto parts store (there are literally thousands of them across the US), Martin Senior is the brand name of their automotive paint and related finish supplies. They have several types of Bondo for different applications. Microlight is a specific product with easy sanding properties.

From contributor N:
I use Bondo to rough it in, then to smooth it out, I use acrylic filler (it comes in a tube like toothpaste). It dries fast and sands smooth quickly.

From contributor V:
If you're doing nail holes in small volume, the Minwax High Performance Filler is very close to Bondo, except that it seems like it uses sawdust instead of lightweight microbeads for filler in a polyester resin (smells like Bondo or fiberglass polyester resin, but comes in small cans). Doesn't really shrink except for once I filled a one-inch wide by one-inch deep hole, and a month later a tiny crescent printed through, if you knew where to look under metallic paint and high gloss lacquer.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor D:
My house was prepped and painted over a six week period from Aug-Sep '09. Two sides of the redwood-clad house (built in 1923) needed to be almost completely sanded down to the wood and needed lots of work. The painter used at least 10 gallons of auto bondo (Evercoat White Star and Evercoat Lite-Weight) on the house. We just had a huge downpour of rain for a few days (mid Oct '09) and I noticed that the paint was puffing up in places. I popped a few of these bubbles and there was water inside. The paint pulled off with a thin coat of bondo in these spots showing the white primer coat below. So, I'm not so sure auto bondo is the best thing to use other than in areas of deep fill. I'm going to have the painter come back and try to fix things. It's such a shame after all the wonderful prep work (and money).