Putty is a popular solution in many products surface coating technologies. Especially for automotive and carbon fiber products like bumper, helmet & related parts. In order to properly apply putty, there’s strict process to follow and checkup. Following we will share a sample of the putty process used in automotive parts coating and also trouble-shooting of common problems.
Surface Requirements for Putty
The primer on the workpiece to be applied must be completely dry and free of oil, dust, and other contaminants. If the primer is not completely dry, the solvent in the primer will not be able to evaporate after the putty is applied and cured, becoming trapped beneath the putty layer. This can easily cause bubbles and delamination when exposed to high temperatures.
Mixing and Applying Putty
Putty: Mix according to the product instructions. The base putty and curing agent must be evenly mixed before application. The typical ratio of curing agent to base putty is 2:100.
1. Applying the First Coat of Putty
The purpose of the first coat of putty is to fill larger indentations. Use a wide, hard scraper to apply the putty. Aim for smoothing, not smoothing. Avoid repeated back-and-forth scraping, as this can clog the pores in the putty and prevent it from drying out for a long time, hindering application. When applying the putty, hold the scraper at a 60° angle to the surface and apply in a slightly curved pattern. After drying, sand to remove loose dust before proceeding to the next coat of putty.
2. Applying the Second Coat of Putty
The second coat of putty is primarily intended to fill in low spots. The thickness of the application should be less than the first. Use a hard scraper for flat surfaces and a rubber scraper for curved areas. When applying, follow the contours of the product, working from right to left and top to bottom. To minimize joints, extend the application as much as possible. After drying, sand to remove loose dust before proceeding to the next coat of putty.
3. Applying the Third Coat of Putty
The third coat of putty is primarily intended to remove pores, sandpaper marks, and minor depressions left by the previous coat. Use a flexible scraper and apply pressure to ensure a smooth, minimally porous layer. After drying, sand until smooth before proceeding to the next coat of putty.
4. Applying the Fourth Coat of Putty
The putty should be diluted and applied thoroughly, ensuring a thin, smooth, and even layer of putty is applied to the entire exterior surface. Only parts that have passed the above treatment can proceed to the next step—the mid-coat. Improper use of putty can cause coating problems. Topcoat is the final step in the automotive painting process.
Analysis of Putty Problems
Blistering after Baking Topcoat
After bubbles form in the topcoat, scrape them away to determine which layer the bubbles are located. If the bubbles appear in the primer layer, the putty and primer bond well, suggesting inadequate substrate preparation. If, after scraping away the bubbles, a layer of primer is found on both the putty and the substrate, the primer may be too thick or the putty was not completely dry before the application.
Pinholes and Depressions after Baking Topcoat
First, consider whether there are any quality issues with the putty itself that could cause pinholes. Secondly, if pinholes occur despite satisfactory putty quality, it is generally due to poor putty application and sanding techniques, resulting in inadequate treatment of blisters. Additionally, improper scraping tools can also cause pinholes during the coating process.
“Prickly heat” after topcoat baking
First, the climate is too humid, and the water absorption rate of the putty is too high. The prepared putty absorbs a considerable amount of water. If the topcoat is sprayed before the water evaporates, a trace amount of water is trapped under the paint film. During heating, the water volume expands rapidly, breaking through the paint film and forming “prickly heat.” Second, if the putty water absorption rate is acceptable, it is dried with hot air and then sprayed with topcoat. If “prickly heat” appears after baking, it is likely that the topcoat was placed in the drying room after spraying before the solvent fully evaporated. As a result, the topcoat surface quickly seals, and the residual solvent is forced to break through the paint film and form “prickly heat.” Of course, it may also be due to the quality of the topcoat, or the polarity of the solvent is not compatible with the topcoat, causing “prickly heat.”
