How to determine if a paint can is completely clean?

1. Visual Inspection
1. No Surface Residue
After cleaning, the interior of the can should be free of paint lumps, stains, or sticky material. Metal cans should reveal their original color (e.g., silver or iron gray), while plastic cans should be transparent or have their original color restored.
2. White Cloth Wipe Test
Wipe the interior with a white cotton cloth. If there is no stain or only a very faint stain (e.g., a light yellow stain is permitted for aviation kerosene cans), the can is considered qualified.
2. Physical Inspection
1. Finger Press Test
For coated cans, press the interior with a finger. If there is no stickiness or indentation, indicating complete cure.
2. Acetone Wipe Test
Wipe the interior with a cotton cloth dipped in acetone. If the cloth does not turn yellow after 3-5 wipes, there is no residual organic matter. III. Chemical Residue Testing
‌1. Solvent Extraction Method‌
Treat suspected residue areas with dilute acid. After centrifugation, test the supernatant for heavy metal content (such as lead and mercury) in accordance with GB 18581-2020.
‌2. Spark Detection‌
Metal tanks can be scanned with a 4V/m voltage probe. The absence of sparks indicates the absence of conductive residue.
‌IV. Industrial Standards‌
‌Oil Tank Cleaning Specifications: The tank must be free of residual oil and water, and the oil and gas concentration must be below the lower explosion limit (LEL) by 4%.
‌Special Vehicle Washing Standards: Aviation kerosene tanks must meet the "Four Free and One Test" standard (free of oil, dirt, fibers, rust, water, and no black residue when wiped with a white cloth).