A powder finish can look good coming out of the oven and still be poorly cured, too thin or poorly bonded. The eye is not enough. So the industry relies on a few standardized tests. Here are the most common ones and what each one tells us.
Thickness: the first check
It is the simplest and most useful measurement. A magnetic or eddy current gauge, used according to the ASTM D7091 standard, reads the film thickness without damaging it, as DeFelsko explains. Technical data sheets generally aim for 1.5 to 3 mils, or 40 to 75 µm, and a little more for outdoor use, according to Products Finishing. Too thin, the film protects poorly; too thick, it loses appearance and flexibility.
Curing: metal temperature and time
A powder only cures fully if the metal itself reaches the right temperature for the prescribed time. A heavy part heats up more slowly than sheet metal: it is the temperature of the part that counts, not that of the air in the oven. The quick check is to rub the finish with a cloth soaked in solvent. Products Finishing describes the method, and points out its limits: it indicates a problem, but only a laboratory test, differential scanning calorimetry (DSC), puts a number on the degree of cure.
Adhesion and impact
The cross-hatch test, ASTM D3359, is known to every painter: you cut the film in a grid, apply adhesive tape, pull it off and rate what remains, from 0B to 5B. The impact test, ASTM D2794, drops a weight on the part to see whether the film cracks. These tests destroy the surface tested: they are done on a test panel coated at the same time as the parts.
Salt spray, to be read with caution
The ASTM B117 test exposes panels to a 5% salt water fog, held at 35 °C, for hundreds of hours. It is used to compare systems with one another. However, the standard itself warns that its results rarely match actual performance in the field. A thousand hours of salt spray does not mean ten winters.
If your project requires a specific thickness or test, write it in your quote request: it is at the start that test panels and checks are planned.
In summary
- Thickness is measured without damaging the finish: 40 to 75 µm in most cases.
- Curing depends on the temperature of the metal, not that of the oven air.
- The cross-hatch and impact tests are done on test panels.
- Salt spray is for comparing, not for predicting a service life.
