Bondo

What Is Bondo?

Bondo is a well-known brand of two-part polyester filler and related repair products made by 3M. The core product is a paste-like filler that you mix with a small amount of cream hardener. Once mixed, a chemical reaction begins and the filler quickly firms up, allowing you to shape and sand it into a smooth surface that can be primed and painted. Although many people associate Bondo with auto body work, it’s also used in home projects for repairing wood, metal, fiberglass, and certain rigid plastics. Think of it as a fast-curing, sandable patching compound for non-structural repairs.

Common Uses in DIY and Home Improvement

• Smoothing dents and dings in car panels before priming and painting
• Repairing chipped or rotted sections of exterior wood trim, sills, and doors (using a wood-specific formulation)
• Filling holes in metal items like mailboxes and tool cabinets
• Rebuilding damaged corners on furniture, laminate countertops, and MDF shelving
• Patching small holes in fiberglass tubs, boat parts, and fairings
• Leveling low spots before finishing or repainting

Note: Bondo-type fillers are cosmetic and non-structural. They shouldn’t replace wood or metal where strength is required, and they aren’t for constant immersion or below-grade wet locations.

Types and Variations

Bondo is a brand umbrella with several filler options. Choosing the right one helps your repair last longer and finish smoother.

• Standard Body Filler: The go-to for smoothing dents and filling shallow imperfections on metal and fiberglass. Sands easily and finishes well under primer and paint.
• Professional or Lightweight Fillers: Similar to standard filler but creamier and easier to sand. Often chosen for finer finishing work.
• Glazing and Spot Putty: A thinner product used after the main filler to fill pinholes, scratches, and minor low spots. Some are one-part air-dry; others are two-part for faster curing and less shrinkage.
• Fiberglass-Reinforced Filler (Bondo Glass): Contains chopped fiberglass strands for extra strength and better bridging across small holes or rust-outs (after cutting away weak metal). Not as easy to sand as standard filler but stronger.
• Long-Strand Fiberglass Filler (Bondo Hair): Even stronger and better for spanning gaps, but rougher to sand; typically used as a base layer followed by regular filler for a smooth finish.
• Bondo Wood Filler: A two-part polyester wood repair filler formulated to bond to wood and take paint. It can accept stain, but color matching is inconsistent; paint usually gives the best result.
• Fiberglass Resin and Mat: Used with fiberglass cloth or mat for structural repairs on fiberglass parts. Often followed by a skim coat of filler to smooth the surface.

How to Choose the Right Product

• Substrate: For metal and automotive panels, use body filler. For wood, use the wood filler. For holes or edges that need reinforcement, pick fiberglass-reinforced filler first, then switch to standard filler for final shaping.
• Size of Damage: Small pinholes and scratches are best handled with glazing putty. Larger dents and voids call for standard or reinforced filler.
• Location and Moisture: Exterior wood can be repaired with wood filler, but prime and paint well. For constantly wet areas or below-grade repairs, consider an epoxy system instead of polyester filler.
• Working Time: Polyester filler typically has 3 to 5 minutes of workable time after mixing at room temperature. In hot weather, choose smaller batches or a slower hardener if available.

Step-by-Step: Using Bondo Successfully

1) Prep the surface
- Remove loose paint, rust, rot, and crumbly material. Cut out weak metal around rust holes rather than covering it.
- Degrease with a solvent approved for the surface (wax and grease remover for auto panels; denatured alcohol for many household surfaces). Let it dry.
- Roughen the area with 80-grit sandpaper to give the filler tooth.
- For wood rot, first apply a wood hardener or consolidant, let it cure, then apply the filler.

2) Mix correctly
- Typical ratio: a golf-ball sized portion of filler to a 1 to 1.25 inch bead of cream hardener (about 3 to 4 percent by volume). Check the label for your exact product.
- Mix on a clean, non-porous board or mixing pad. Fold and smear until the color is uniform with no streaks. Work quickly.

3) Apply in thin layers
- Press the filler firmly into the surface to drive out air and improve adhesion.
- Build up in layers no thicker than about one quarter inch each. Allow each layer to set before adding more.
- Feather the edges slightly beyond the repair area so the transition sands smooth.

4) Shape and sand
- At the green stage (partially set, about 5 to 10 minutes), use a body file or rasp to quickly knock down high spots.
- Once fully set (typically 15 to 20 minutes at 70 to 75 degrees Fahrenheit), sand with 80-grit to shape, then refine with 120 or 150, and finish with 180 to 220 grit.
- If pinholes appear, apply glazing putty, let it cure, and sand smooth.

5) Seal and finish
- Blow off dust and wipe with a suitable cleaner.
- Apply a primer-surfacer to seal the repair and fill micro-scratches. Block sand if needed for a flat finish.
- Topcoat with paint that suits the substrate and location. For exterior wood, use a quality exterior primer and paint for moisture protection.

Safety and Storage

• Ventilation: Polyester fillers release vapors during mixing and curing. Work in a well-ventilated area.
• PPE: Wear nitrile gloves, eye protection, and a respirator with organic vapor cartridges during mixing. When sanding, use a P100 or N95 dust mask at minimum; a combination cartridge is ideal.
• Fire: Keep away from flames or sparks. The materials are flammable until cured.
• Storage: Keep containers tightly sealed in a cool place. Avoid prolonged heat and direct sun. If the filler thickens or the hardener separates or crystallizes, performance will suffer; replace if in doubt.

Common Mistakes to Avoid

• Skipping surface prep or bonding over rust, dirt, or glossy paint
• Over- or under-catalyzing (too much hardener can cause cracking and pinholes; too little leaves the filler gummy)
• Mixing big batches you can’t spread within a few minutes
• Applying a thick blob instead of building up in thin layers
• Overworking the filler after it starts to gel, which pulls the surface and adds air
• Neglecting primer before paint, leading to poor adhesion and telegraphed scratches
• Using polyester filler for structural repairs or places that stay wet

Related Terms

• Body filler
• Glazing putty
• Fiberglass resin, mat, and cloth
• Epoxy filler and wood consolidant
• Primer-surfacer and sealer
• Body file (cheese grater), sanding blocks, spreaders

Practical Examples

• Exterior window sill repair: Consolidate soft wood, apply Bondo Wood Filler in layers, sand smooth, prime, and paint to shed water.
• Auto panel dent: Hammer and dolly to reduce the dent, skim with body filler, block sand, glaze pinholes, then prime and paint.
• Rust hole in a metal mailbox: Cut out weak metal, back with fiberglass mat and resin or use fiberglass-reinforced filler, then apply standard filler on top for a smooth finish.
• Damaged cabinet corner: Rebuild the missing corner with standard filler, shape at the green stage, sand, and paint.
• Fiberglass tub chip: Clean, scuff, fill with a small amount of filler or a tub repair kit, sand, and touch up with compatible coating.

Bondo products help you create paint-ready surfaces quickly. Choose the right variant, prep carefully, mix accurately, and finish with proper priming and paint for a repair that looks good and holds up.