How to fill bolt holes in car body
The Art of Filling Bolt Holes: A Guide to Restoring Your Car’s Body
Imagine this: you’re finally getting around to restoring your beloved classic car. You’ve spent countless hours scouring junkyards and online marketplaces for missing parts, meticulously cleaning and prepping the body, now you’re ready for paint. But then, a chilling realization hits you: those pesky bolt holes left behind from old, removed accessories, or even from the car’s original manufacture, are staring back at you, threatening to mar the perfect, smooth finish you’ve worked so hard to achieve. Don’t despair! Filling bolt holes is a common and relatively simple task that, when done right, will leave your car looking as good as new.
This guide will walk you through the process of filling bolt holes in your car’s body, covering everything from choosing the right materials to applying the finishing touches. We’ll delve into different methods, tips for achieving a professional finish, and common mistakes to avoid. By the end, you’ll be equipped with the knowledge to transform those unsightly holes into a seamless part of your car’s body.
Understanding Bolt Hole Types and Locations
Before diving into filling techniques, let’s take a moment to understand the diverse types of bolt holes you might encounter on your car’s body. These holes can be found in various locations, each presenting its own set of challenges. Here’s a breakdown:
1. Accessory Mount Holes:
These are often the easiest to deal with. They were once used for attaching items like bumpers, mirrors, license plate brackets, and other non-structural accessories. They are usually smaller in diameter and are often located on relatively flat surfaces.
2. Structural Bolt Holes:
These holes are more intricate and often require a more careful approach. They were used for attaching structural components like suspension parts, engine mounts, or chassis reinforcements. They are typically larger in diameter and can be located in areas with complex curves or angles, making them harder to fill seamlessly.
3. Factory Bolt Holes:
Many cars come with factory-installed bolt holes that were used for attaching panels, trims, or other parts during assembly. These holes can be small and shallow, often hidden beneath trim or other components, and often require special attention during restoration to maintain the car’s originality.
Choosing the Right Filling Material
The next crucial step is selecting the right filling material. A variety of options are available, each with its own pros and cons. Here’s a guide to help you choose the best material for your specific needs:
1. Body Filler (Bondo):
Body filler, commonly known as Bondo, is a popular choice for filling larger holes and imperfections. It’s a two-part epoxy putty that mixes to form a thick, moldable paste. Bondo is known for its strength, durability, and ease of use. It can be sanded and shaped to achieve a smooth, even surface.
2. Plastic Welding:
This method involves using a heated plastic rod to fill the hole and melt it into the surrounding plastic material. It’s often used for filling holes in plastic bumpers, trim pieces, and other plastic components. Plastic welding offers a strong and durable bond that’s often invisible.
3. Epoxy Putty:
Epoxy putty is a two-part compound that offers excellent adhesion and strength. It’s a versatile option for filling small holes and imperfections in both metal and plastic. Epoxy putty can be sanded and primed, creating a smooth surface for painting.
4. Soldering:
For smaller holes in thin metal panels, soldering is a viable option. It involves melting a solder alloy and flowing it into the hole, creating a strong and permanent bond. Soldering is often used to repair damaged wiring or fill small holes in sheet metal.
5. Rivets:
Rivets are metal fasteners that are used to join two or more pieces of material together. They can be used to fill holes by inserting a rivet into the hole and then expanding the rivet head. Rivets are a quick and easy way to fill holes, but they may not be suitable for all applications.
Preparing the Bolt Hole for Filling
Once you’ve chosen the appropriate filling material, it’s time to prepare the bolt hole for filling. Proper preparation is crucial for achieving a smooth and lasting finish:
1. Clean the Hole Thoroughly:
Start by thoroughly cleaning the hole with a wire brush or a scraper to remove any rust, dirt, or debris. This ensures that the filling material will bond properly to the surrounding metal. Use a vacuum cleaner to remove any loose particles.
2. Degrease the Area:
After cleaning, degrease the hole and surrounding area with a solvent like acetone or mineral spirits. This removes any oils or contaminants that could interfere with the bonding process.
3. Prime the Hole:
Depending on the filling material you use, you may need to prime the hole before applying the filler. Priming helps create a better adhesion surface for the filler. Use a primer that’s compatible with the filling material and the surrounding metal.
Filling the Bolt Hole with Chosen Material
Now that the hole is prepped, it’s time to apply the filling material. Here’s a step-by-step guide for each method:
1. Using Body Filler (Bondo):
1. **Mixing:** Mix the Bondo according to the manufacturer’s instructions. Use a mixing stick to thoroughly combine the resin and hardener. Work quickly, as Bondo starts to harden rapidly.
2. **Applying:** Use a putty knife to apply the Bondo to the hole. Apply it in thin layers, working it into the hole. Ensure that you completely fill the hole and create a smooth, even surface.
3. **Shaping:** Once the Bondo has started to harden, use a putty knife or sanding block to shape the filler to the contour of the surrounding metal.
4. **Drying:** Allow the Bondo to dry completely before proceeding to the next step. The drying time will vary depending on the temperature and humidity.
5. **Sanding:** Once dry, sand the Bondo with progressively finer grades of sandpaper, starting with a coarse grit to remove any excess material and smooth out any imperfections. Finish with a fine-grit sandpaper to achieve a smooth, even surface.
2. Plastic Welding:
1. **Prepare:** Remove any loose or broken plastic pieces around the hole. Clean the area around the hole using a scraper or wire brush, followed by a degreasing agent.
2. **Heating:** Heat the plastic welding rod using a heat gun or a special plastic welding tool. The rod should be heated to a molten state, but not so hot that it burns or drips.
3. **Applying:** Carefully apply the molten plastic rod to the hole, working it into the surrounding plastic. Use a small scraper or a putty knife to spread the molten plastic evenly, ensuring it bonds well to the existing plastic.
4. **Shaping:** While the plastic is still soft, use a scraper or a sanding block to shape the filled area to the contours of the surrounding plastic.
5. **Cooling:** Allow the plastic to cool completely before proceeding to the next step.
6. **Finishing:** Sand the filled area with progressively finer grades of sandpaper to achieve a smooth, even surface.
3. Using Epoxy Putty:
1. **Mixing:** Mix the epoxy putty according to the manufacturer’s instructions. Use a mixing stick to thoroughly combine the two parts.
2. **Applying:** Use a putty knife to apply the epoxy putty to the hole, working it into the hole and creating a smooth, even surface.
3. **Drying:** Allow the epoxy putty to dry completely before proceeding to the next step. The drying time will vary depending on the type of epoxy putty and the ambient temperature.
4. **Sanding:** Once dry, sand the epoxy putty with progressively finer grades of sandpaper to achieve a smooth, even surface.
4. Soldering:
1. **Clean:** Thoroughly clean the hole and surrounding area using a wire brush and a degreasing agent.
2. **Flux:** Apply flux to the area to promote the flow of solder and prevent oxidation.
3. **Soldering:** Heat the hole using a soldering iron. Apply solder to the heated area, allowing it to flow into the hole and fill it completely.
4. **Cooling:** Allow the solder to cool completely before proceeding to the next step.
5. **Shaping:** Use a small file or sandpaper to shape the soldered area to the contour of the surrounding metal.
5. Using Rivets:
1. **Drilling:** Drill a hole slightly larger than the rivet diameter in the metal panel.
2. **Inserting:** Insert the rivet into the hole.
3. **Expanding:** Use a rivet gun or
