Damage that went unnoticed under the previous coating is exposed after the coating is stripped away in a blasting cabinet.
TORONTO, Ont. — Nasty-looking wheels, all covered with rust and cracked paint, are pretty well an invitation to a roadside DOT inspection. Rusty wheels may not reflect the attention to detail your fleet usually brings to the maintenance game, but they don’t send the right message to the creeper cops. For about half the cost of a new steel wheel, refinishing is a cost-effective alternative to new wheels.
Simply brush-painting the wheels is not recommended because it can hide defects such as cracks or deep corrosion that would be revealed by stripping the current coating off the wheel and applying a new finish. Many commercial tire shops offer refinishing services. Last year, Kal Tire alone refinished about 50,000 wheels in its three shops that offer the service.
“It’s an attractive option for larger fleets that can take tires and wheels out of service for a short period of time,” says Norm Schmidt, director of retreading at Kal Tire in Chilliwack, B.C. “It’s harder for smaller fleets because they often don’t have tires and wheels in inventory, but we sell new wheels, too.”
Schmidt says the refinishing process begins with an inspection for obvious defects, like cracks, elongated bolt holes, bends along the flange, or an out-of-round condition. If the wheel passes preliminary evaluation, its placed in a blasting cabinet where the previous coating is removed, and the wheel cleaned and degreased using media such as steel shot, sand or glass beads of varying sizes.
Special tools can measure coating thickness. If the coating thickness exceeds 3.5 thousandths of an inch, the clamping force of the wheel nuts can be compromised.
Shorten the blasting process
“The media is chosen for coating removal and paint preparation,” he says. “You can shortcut the process by using larger, more aggressive media, but you run the risk of pitting the wheel or possibly peening thin cracks closed. Use too fine a media, and the process takes longer.”
Some shops use a chemical bath to dissolve the coating, others may use a bake oven. Both are fine as long as the wheel isn’t damaged in the process.
The wheel is visually inspected again while stripped to the bare metal prior applying the electrostatic powder coating. The powder is positively charged and air-sprayed onto the wheel, which is negatively charged, causing the powder to cling to the metal. This step required the coating to be applied evenly, taking care to keep the coating thickness to a maximum of 3-3.5 thousandths of an inch thick.
“The coating cannot be any thicker, especially on the mounting surfaces and between the bolt holes,” Schmidt warns. “Excessive coating thickness can lead to a loss of clamping force and eventually loose nuts and possibly a wheel separation.”
After the coating is applied, the wheel is cured in an oven for 15 minutes to a temperature of 380 to 420 Fahrenheit. This liquifies the coating and bonds it to the wheel. The wheel cools before a final inspection that includes testing for coating thickness.
“Some shops are now offering a zinc undercoating for additional protection against corrosion,” Schmidt says. “There’s a limit to how may times wheels can be refinished because each time it goes through the blaster to remove the old finish, some of the metal comes away, too. The undercoating can extend the miles before it starts looking scabby again.”