Not all materials can be printed on directly. Products made of different materials require different pre-treatment before printing.
First, for glass and ceramic materials: Use isopropyl alcohol to thoroughly degrease the surface, removing dust and fingerprints, and ensure it is completely dry. Note that you need to apply a UV coating specifically designed for glass and ceramics. Wait for the coating to dry before printing — if the coating is not dry, the print is prone to wrinkling.
Second, for plastics such as PP and PE (low-surface-energy materials): A special primer or coating agent is required. Without a coating, printing directly on them will definitely not adhere. Note that the coating should be applied evenly — too much coating will easily cause wrinkling. After applying, let it sit for 5 to 10 minutes before printing.
Third, for metals such as stainless steel and aluminum alloy: First, remove oil and rust. Use a metal cleaner or lightly sand with sandpaper to increase surface roughness. Some metal types may require a metal-specific coating. Note to avoid any residual moisture. Print immediately after drying to prevent secondary oxidation.