Step tests are a way for D2 printers to determine the correct amount of pretreatment a garment needs. They're conducted on a shirt where different amounts of pretreatment are applied evenly across the fabric in rows or columns. The first step has less pretreatment than the second, and the second less than the third.
In this example (left), we did multiple steps in variations of 4 grams per step. We utilized a pretreatment machine and covered most of the shirt except for the far right of the fabric, sprayed the shirt, marked the end point of the spray, and then moved the cover sheet to the left approximately 1.5–2" and resprayed the garment. This was repeated seven times resulting in rows of pretreated areas that had 0, 4, 8, 12, 16, 20, 24, and 28 grams of pretreatment applied.
The pretreatment was then heat set and printed with two different white boxes that stretched across the shirt. A single and double pass of white ink was printed to show the differences two levels of white ink would have on the underlying pretreatment. As seen in the picture, it is easy to see the different zones and how the white ink shows on the step test areas.
In the step test, you are looking for the point where any additional pretreatment does not increase the smoothness or whiteness of the printed white ink. For this specific shirt, an obvious step is seen in each area from 0 grams through 20 grams of pretreatment. Somewhere between 20 and 24 grams of pretreatment, it is tough to determine if there is any improved print quality. There is no optical advantage to applying more pretreatment at this point. Using more than 20–24 grams of pretreatment is more pretreatment than necessary. This will result in spending more money on pretreatment than necessary without improving the quality of the white ink film.
A step test is a beneficial tool when printing with a D2 machine. Various shirt colors, brands of shirts, types of fabric, and other variables affect how much pretreatment is necessary to achieve these results. My typical response is that a lighter weight shirt will require less pretreatment than a heavier weight shirt and a lighter colored shirt requires less than a darker colored shirt. All these variables are sliding in nature, resulting in a myriad of results that vary based on the inputs.
Source: Brian Walker, Image Armor/RTP Apparel