Elena Rossi
Detecting Bondo, or body filler, beneath a layer of paint is a common concern for car buyers, especially when purchasing a used vehicle. In the past, the knuckle test or magnet test were used to detect repairs and assess paint finish quality. However, these methods are unreliable, especially with modern cars made from various materials like aluminium, steel, and plastic. Today, paint depth gauges, also known as paint meters or coating thickness testers, are the preferred method for accurately determining paint thickness and detecting underlying repairs. These gauges use ultrasonic or magnetic technology to measure the thickness of paint layers and can help buyers make informed decisions when purchasing used vehicles.
| Characteristics | Values |
|---|---|
| Traditional method of detecting Bondo | "The knuckle test" or "the magnet test" |
| Use of "the magnet test" | Rely on magnets to reveal where fillers have been added for bodywork repair |
| Limitations of "the magnet test" | Only helpful in testing the steel portions of the vehicle |
| Modern methods of detecting Bondo | Coating thickness gauge, paint meter, paint depth gauge, paint thickness gauge, refinishing gauge, dry film thickness gauge, mil gauge, banana gauge, spot checker |
| Function of coating thickness gauge | Measure the thickness of different types of coated materials (usually metal) using an ultrasonic signal that is converted into a thickness measurement usually denoted in "mils" |
| Cost of coating thickness gauge | $20-$800 |
| Accuracy of coating thickness gauge | ±5% |
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What You'll Learn
Using a paint thickness gauge
The cheaper options tend to be mechanical gauges, which use a permanent magnet, a calibrated spring, and a graduated scale. They measure the force required to pull the magnet off the coated surface to determine thickness. These options are great for detecting Bondo or other fillers, but they may not be as accurate as more expensive options, with accuracy typically within ±5%.
More expensive options tend to be electronic gauges, which use a constant pressure probe to provide consistent and accurate readings. These readings are shown on an LCD display and can be stored and analysed instantly.
When using a paint thickness gauge, it is important to determine the expected paint thickness of the vehicle and then measure the actual thickness. This can help identify if there has been any repainting or if there are variations in the paint thickness, which could indicate repairs.
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Performing a magnet test
To perform the test, you will need a magnet that is strong enough to stick to the metal body of the car through the paint. A small refrigerator magnet or a magnetic business card can be used for this purpose. Wrap the magnet in a thin piece of cloth or paper towel to prevent scratching the paint. Drag the magnet along the car's surface, and if it falls off or fails to stick in certain areas, it may indicate the presence of Bondo or body filler.
It is important to note that the magnet test has limitations. It may not work on modern cars made of materials like aluminium or damage-resistant plastic. Additionally, it cannot detect if entire panels have been replaced. The test is also dependent on the strength of the magnet and the thickness of the paint and body filler.
While the magnet test can provide some indication of repairs, it may not always be accurate. For a more precise assessment, it is recommended to use a specialised tool like a paint thickness gauge or a Bondo detector magnet tool, which can measure the thickness of the paint and body filler more accurately.
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Inspecting the car for signs of repainting
Inspecting a car for signs of repainting is a crucial step in ensuring you're getting a vehicle in good condition and understanding its history. Here are some detailed instructions on how to inspect a car for repainting:
Start by looking for inconsistencies in the paint's colour and texture. A repainted car may show variations in colour tone between different panels. For instance, a fender that appears slightly darker than an adjacent panel could indicate a repaint. Keep in mind that paint colour matching is challenging, especially for older models, as UV rays cause the paint to fade over time.
Examine the trim around windows and bumpers, as well as underneath the car. Spray paint can inadvertently reach these areas. Signs of paint on suspension components like control arms and anti-roll bars could indicate a previous paint job. A tape line on a doorjamb or painted nuts and screws are also red flags.
Use a paint thickness gauge to check for differences in paint depth between panels. A variation of 40 mils or more between two panels may suggest that one panel has been repainted. More expensive gauges can cost up to $2,000 and use sonar to reveal the thickness of various paint layers.
Alternatively, try the "magnet test" by using a small refrigerator magnet to detect where fillers have been added for bodywork repair. If the magnet doesn't stick to the sheet metal, it indicates that repairs have been made. However, this method only works on steel portions of the vehicle, as many modern cars are made of multiple materials, including plastic bumpers.
Finally, look for signs of oversprays, which can appear as dull patches with rough surfaces compared to the rest of the car. Body shops use masking tape to cover areas they don't want to paint, and improper removal can leave visible tape lines or abrupt depth transitions.
Remember, inspecting for signs of repainting can help you make an informed decision about the vehicle's history and any potential underlying issues.
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Using a paint meter
A paint meter is a handheld device used to measure the thickness of paint on a surface. It is an essential tool for anyone looking to buy an old car, as it can help detect any repairs or bodywork done to the vehicle.
There are two main types of paint meters: mechanical and electronic. Mechanical gauges use a permanent magnet, a calibrated spring, and a graduated scale to measure the thickness of the paint. The user holds the magnet against the surface and pulls it away, then uses the scale to determine the thickness of the paint. This type of gauge is typically priced at around $350 and has an accuracy rate of ±5%.
Electronic paint meters, on the other hand, use a constant pressure probe to provide consistent readings. These readings are displayed on an LCD screen and can be stored for future reference or analysis. They are much more popular in the automotive detailing industry due to their accuracy and ease of use.
When using a paint meter to detect Bondo, it is important to take multiple readings from different areas of the car. Consistency in the thickness of the paint is key; if one area shows significantly different readings from the others, it may indicate that Bondo or another filler has been used. Additionally, areas of reduced thickness may indicate potential rust or other damage.
It is also important to note that not all paint meters can detect paint thickness on all types of materials. For example, many magnetic paint meters are unable to measure paint thickness on aluminium. As such, it is important to select a paint meter that is specifically designed for the type of surface you are testing.
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Checking for uniformity in paint thickness
When it comes to checking for uniformity in paint thickness, there are several methods that can be employed. One common approach is the use of a paint thickness gauge, which provides a simple and effective way to assess the quality and uniformity of coatings. These gauges can be placed flat against a surface to measure paint thickness, helping to ensure uniform application across an area. They are especially useful for verifying the quality of work after a surface has been treated or repaired, as inconsistent coating can impact durability, appearance, and performance.
Paint thickness gauges come in a variety of price ranges and types, including ferrous (magnetic) gauges specifically designed for ferrous metals like steel. More expensive options, such as the Defelsko Paint Thickness Gauge, use sonar to show the thickness of various paint layers. These advanced gauges can be useful for professionals who need precise measurements to work with.
Another method for checking paint thickness is the traditional "knuckle test," which involves rapping on the metal surface to identify areas with different thicknesses or "dead spots." A similar approach is the "magnet test," where a magnet is used to identify areas where repairs have been made, as the magnet will not stick to metal that has been filled or repaired. However, this method is limited to steel portions of a vehicle and is not effective on other materials like aluminium or plastic.
For those looking to measure paint thickness on drywall, ultrasonic paint thickness gauges offer a non-destructive way to measure the total thickness of the paint system. The PosiTector 200 B1, for example, can measure the combined thickness of primer and paint layers on drywall, which typically has three layers. This method is simple and does not require calibration adjustment for most applications.
In conclusion, checking for uniformity in paint thickness is important for maintaining product quality, preventing corrosion, and ensuring compliance with industry standards. By using tools like paint thickness gauges or employing traditional methods like the knuckle or magnet test, individuals can assess the uniformity of paint thickness and make informed decisions when purchasing used vehicles or evaluating repairs.
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Frequently asked questions
Use a paint thickness gauge to measure the paint depth. If the instrument does not give a measurement, it could mean that the thickness is over the gauge's limit and there may be Bondo or filler present.
It is a tool used to measure the thickness of paint on a car.
There are two types of paint thickness gauges: mechanical and electronic. Mechanical gauges use a permanent magnet, a calibrated spring, and a graduated scale to measure the force required to pull the magnet off the coated surface. Electronic gauges use a constant pressure probe to provide consistent readings that are displayed on an LCD screen.
Paint thickness gauges can range in price from $20 to $2,000 depending on the quality and features offered.
Yes, one common method is to use a magnet. Since steel is magnetic, you can use a magnet to test the thickness of paint over steel. If the magnet does not stick to the sheet metal, it may indicate that there have been repairs done in the past. However, this method only works on steel portions of the vehicle.
