Building Prototypes Dan Gelbart part 6 of 18 Coatings

Summary

Dan Gelbart explains how to prepare surfaces for reliable painting and adhesive bonding. He also demonstrates electrostatic powder coating and a method for repairing dents before painting.

Sandblasting and Surface Preparation

• A sandblaster is an inexpensive, useful tool for preparing metal before painting or bonding.
• Gelbart recommends replacing the usual window and sacrificial plastic cover with a diamond-coated glass panel, such as those used in supermarket checkout scanners. In his experience, it resists scratching and avoids frequent replacement.
• A vacuum helps control dust and makes it easier to see the work.
• Surface adhesion depends on both mechanical roughness and chemical interaction. Gelbart argues that solvent cleaning alone is often insufficient: it can dilute oils but may leave a molecular film that interferes with bonding.
• To check whether a surface is ready, use the water-wetting test:
  • On a poorly prepared surface, water beads up.
  • On an activated surface, water spreads and stays put.
  • The droplet’s contact angle indicates surface energy. Better wetting suggests a higher-energy surface suitable for bonding or painting.
• Gelbart prefers sandblasting. When that is impractical—such as with large objects or thin sheet metal that could warp—he recommends either heating the surface to roughly 150–300°C or scrubbing it with a powdered bathroom cleaner, such as Ajax or Comet, and then rinsing it.
• Activated surfaces gradually lose their improved wetting properties as airborne hydrocarbons settle on them. Painting or bonding should follow within about an hour. A “gumming” solution can preserve prepared parts for later use, but Gelbart says it is generally unnecessary for prototype work.

Checking Paint Adhesion

Gelbart demonstrates a crosshatch tape test: score the painted surface into small squares, press strong tape onto it, and peel the tape off. Any lifted paint indicates a failure.

He recommends repeating the test after bending the sample. A coating that initially appears to adhere may detach after bending, indicating that it has not formed a durable bond.

Electrostatic Powder Coating

• Gelbart favors powder coating because it avoids the cleanup associated with wet paint.
• Hang the workpiece from a grounded support. A small hole can be left in a fabricated part for this purpose.
• The coating gun uses low-pressure air and high voltage to attract powder to the grounded object. Electrostatic attraction helps powder reach around edges and coat the back of a part while reducing overspray.
• Basic powder-coating systems are relatively inexpensive. Gelbart suggests keeping separate, inexpensive guns for different colors rather than flushing a gun when changing colors.
• He also demonstrates a chrome-effect powder. It may not look metallic when first applied, but heating causes it to fuse into a durable, chrome-like finish.

Filling Dents Before Coating

For dents in welded sheet-metal enclosures, Gelbart advises against relying on high-temperature patching compounds, which he says can be weak or discolor the finish.

Instead, heat the sandblasted part to the powder-coating temperature and sprinkle the same coating powder into the dents. The powder melts, cures, and fills the low spots. Once the part cools, sand the excess flat before applying the final coating.

Guides and Demonstrations Covered

1. Modifying and operating a sandblaster.
2. Preparing surfaces and checking readiness with a water-wetting test.
3. Testing paint adhesion with a crosshatch tape test, including after bending.
4. Applying electrostatic powder coating and a chrome-effect finish.
5. Using coating powder to fill dents before final painting.

Main speaker/source: Dan Gelbart, presenting workshop demonstrations and recommendations.