Bondo Tips

In this article Jim Hingst provides detailed instructions on how to use 3M brand Bondo polyester putty.

Bondo, a 3M line of polyester putty, is not just for automotive body repair. It is also useful for patching up damaged woodwork and fixing household items in need of repair. A few artists even use it for sculpting caricatures and props. What makes it great for these applications is that it is easy to work with, it cures fast and, best of all, it is cheap.

I am sure that most guys have heard of Bondo and many have worked with it. In a DIYers bag of tricks it is as essential as Duck Tape and WD40. For any of you ladies who have not worked with Bondo, it has a similar appearance and consistency to cake frosting. In my opinion, filling a dent on a car is easier than decorating a cake.

Bondo is so easy to use that some body shop men have belittled its use, referring to it as body repair in a can. Many shade tree mechanics, who have haphazardly slathered it on a dent, have contributed to the bad rap that the product gets. However, used properly, Bondo repair work can look as professional as anything that the body shop can perform. 

Bondo and other automotive putties consist of two parts: a polyester resin and a catalyst hardener. These two parts are typically mixed at a ratio of about 50 to 1. After it is thoroughly mixed, you simply spread it on the damaged area, sand it, prime it and paint it. Here are some tips that you should keep in mind when working with the product:

Bondo Instructions. 

Carefully read the instructions and follow them precisely. This is not a product to get creative. Do not alter the ratio of hardener to resin. If the Bondo that you applied does not harden after 45 minutes, you probably did not use enough hardener or the components were not mixed thoroughly.

Bondo Working Time. 

Bondo and other body fillers cure through a chemical reaction. Bondo cures fast to begin with so you typically only have about 3 to 5 minutes of working time…less if you are working in a warm shop. That’s because as your shop temperature increases, working time decreases.

Mixing Pallet. 

Do not use corrugated board or any other porous material as a mixing pallet. Porous materials will absorb the polyester resin from the filler. I recommend covering your work surface with a smooth plastic application tape, such as RTape AT60. When you finish mixing up body putty you can easily remove the application tape and discard it for fast clean up.

Plastic application tape, such as RTape AT60, is ideal as a surface upon which to mix Bondo.

When you need a clean surface for mixing a new batch of Bondo, discard the used application tape. Clean up is fast and easy.

Batch Size. 

Mix small batches at a time. For example, I mix 1/3 cup of resin with 1 ¼” strip of hardener. Mixing a larger amount of resin and hardener will accelerate the rate of cure.

 1/3 measuring cup contains the equivalent volume of a golf ball, which is the ideal amount of resin that you need for a batch of Bondo. To this resin you will need to mix in about 1 ¼ inch strip of hardener.

Mixing Bondo Components. 

Work fast but make sure that the two components are thoroughly combined. Each component is a different color. Using a plastic squeegee mix the components until the mixture is one uniform color. For mixing Bondo, I use the cheapest vinyl application squeegees that I can find. Of course, if you don’t want to use a squeegee, you can buy plastic body putty applicators.

Using a plastic squeegee thoroughly mix the grey resin and the red hardener until the color of the mixture is one uniform color.

Substrate Preparation. 

Prior to applying Bondo to a surface, remove any paint so the body filler securely bonds to the base substrate rather than bonding to the paint. Rough up the substrate prior to application by sanding it with 80-grit sandpaper. This will give the surface some tooth for the Bondo to bite into.  If you are repairing dented metal, you will need to sand down to the metal. Sand an area around the dent large enough so you can feather the Bondo, blending it into the existing undamaged paint job.

Surface Cleaning. 

After sanding you will need to prep the surface in a manner similar to prepping a vehicle prior to vinyl application. See my article 3-Step Prep. The first step is to wash the surface with a non-abrasive cleaner such as Simple Green. Then wipe the surface down with wax and grease remover. The final step is to clean any remaining residue with isopropyl alcohol  (IPA). View my video covering surface preparation:

Masking Tape. 

Mask off anything that you don’t want the Bondo to  stick to. A little masking tape can save you a lot of clean up time.

Scratch Coat. 

After applying the first  “scratch coats” of Bondo to fill a dent, you can sand it down with 80-grit sandpaper. These first coats should overflow the dent so you have enough applied material to shape it to match the contour of the surrounding area.  When sanding make sure that you wear a proper, tight-fitting dust mask. After sanding wipe the dust off with a rag dampened with IPA.

 After applying the first  “scratch coats” of Bondo, sand it down with 80-grit sandpaper.

Know When to Stop. 

Bondo sets up very quickly after mixing. As soon as it becomes tacky, it is not workable so stop smoothing it with your squeegee. This is why it is preferable to only mix small batches, which are more manageable in application. Only mix enough that you can apply in 30 seconds or less.

Skim Coat. 

Working with Bondo is similar to drywall work. The finishing touch is the skim coat. This  coating will cover the rough coats and extend past them over to the surrounding area. Again you can begin by sanding with 80-grit paper. Then use finer grits to achieve a smoother surface. Complete the job by wet sanding the repaired surface with 400-grit wet/dry sandpaper.

Filling Holes.  

To fill a hole with Bondo, you need to provide some backing. If it is a small screw hole, not much more is needed to support the putty other than a piece of tape. Large holes require more substantial support, such as metal mesh body patch. The Bondo brand of metal mesh, 932 Self Adhesive Patch,  is pressure sensitive and and sticks to bare metal. The 3M Bondo mesh is ideal for  patching a rusty hole in an old beater, bridging the open area, giving the body putty something to stick to. After the mesh is cut to the size needed with tin snips and secured in place, coat the mesh with Bondo. Several coats will likely be needed as you build up the patch. Between each coating, sand the hardened putty with 80-grit sandpaper to remove excess Bondo. The coarse sandpaper will also rough up the surface giving it the tooth needed for the subsequent coating of Bondo to achieve good intercoat adhesion.

3M Bondo Self Adhesive Patch bridges holes giving the body putty something to stick to. 

Guide Coat.

If you want to check the quality of your work, you can dust on a light guide coat of a dark paint color. Subsequent sanding with 180-grit paper will reveal any low spots and surface inconsistencies.

Primer. 

After sanding your repair job, spray two coats of primer. Before painting a finish coat, sand the primer with 400-grit paper.

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.

Slides of Jim's Artwork

© 2015 Jim Hingst