Matt is a professional painter and freelance writer, sharing his knowledge, house-painting tips, and product reviews.
3M Bondo Wood Filler and Hardener
When preparing cabinets and woodwork for paint, you're likely to encounter damaged wood in need of repair. There are many options available for wood repair, but most of the products I've experimented with dry too slow, shrink, or dry too soft.
Bondo wood filler is what I use the most for repairing damaged wood I'm painting, especially cabinets. I've used this two-part filler multiple times to fill large cracks and hardware holes in cabinet doors. The filler dries very hard without cracking.
The filler comes with a small tube of the cream hardener needed to activate the hardening of the filler itself. The more hardener added, the faster the product dries.
This is a very strong wood filler that's worked well for me whenever I've used it, but I'll share a few of my likes and dislikes so you can decide if using Bondo for wood repair is a good choice for your painting project.
Waterproof Wood Filler
The filler is water-resistant and holds up well outside when primed and painted. I've used this product to patch woodpecker holes and cracks in exterior trim without issues. Not all wood filler is suitable for outdoor use. Water-based filler, for example, can fail if exposed to excessive moisture, even when primed and painted. I'd rather use waterproof wood filler outside than one that might fail.
Shrinkage is very minimal compared with water-based products I've used. One coat is usually enough to level shallow holes in wood, but deeper holes require two coats. I always apply two coats when using this product to fill hardware holes in cabinet doors, and this filler works great for that purpose.
Strong Wood Filler
Much like automotive Bondo, this one dries very strong and hard too. The material sticks really well to wood without cracking or falling out of holes, in my experience. Hardness is important for furniture repair, cabinet doors, and exterior woodwork exposed to the elements. The downside is difficult sanding.
Fast Dry Time
The fast drying time can work against you if you have multiple holes to fill. You can add less hardener to extend the drying time, but even then, you have about five to seven minutes to fill the holes before it's too late. I like the fast dry time though for patching deep holes. With a water-based product, or spackle, the material would take a long time to totally dry in a deep hole.
This stuff is probably not the best choice if you're overly sensitive to strong chemical odors. The smell is terrible from the moment the can is opened. You can of course where a respirator or dust mask to avoid direct exposure to the fumes, which is what I do. The odor does go away in about one to two hours after drying. The smell is probably the biggest negative about this product when used indoors.
Bondo dries very hard, which makes it harder to sand than other products I've used. If you over-apply the material, it's very difficult to sand without using an electric sander. I always apply a very thin layer over holes to make sanding easier.
Is Bondo Wood Filler Worth It?
Bondo wood filler works really well for me when I need to repair holes in cabinets, molding, and exterior trim. The main reason I use this product is for its strength, fast drying, and very low shrinkage. I do a lot of cabinet painting, which often involves filling old hardware holes, and with the fast drying of this product, I'm able to fill multiple holes on doors and paint them on the same day. The material is paintable in about 20 to 30 minutes.
This is not a good product choice for doing any grain filling, or patch work that involves carefully smoothing out patches like you would with drywall mud. The product dries too hard and too fast for that. This product is for filling holes, that's it. The strong odor is really bad, but I feel like it's a small price to pay for the excellent filling capability you get.
I've experimented with various products, including Durham's water-based putty, but most of those take too long to dry when patching a deep hole, and those products are susceptible to failure from exposure to moisture.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
Questions & Answers
Question: Do you have to prime and paint if you are using outside?
Answer: You don't have to, but I would prime your Bondo patches because primer helps the patch blend in with the paint so it doesn't flash. Primer also seals over the patch to protect it more for durability.
Question: I'm dealing with an old exterior door that has several hardware holes. My plan is to mix it and then put it in the bottom of a baggie, cut a hole in the corner, and squeeze into the hole — right about that? But should I fill it half and do a second fill? And then maybe, keeping the second fill just shy of the door surface, do a third coat, very smooth? This plan makes sense to me, but how does it seem to you? Any pitfalls?
Answer: I would just mix up a small amount and use a putty knife to patch the holes. You're going to have to apply two or three coats if the holes are deep. This wood filler doesn't shrink much, but you have to build up the layers until it's level with the surface. You could use the bag to squeeze it out, but I think using a putty knife is faster and easier. You have to work fast with Bondo because it hardens in less than 5 minutes.
© 2019 Matt G.