- The material must be absorptive, to perform blotting action
- It must have a fine texture but not be too fine, as this may block imperfections
- For colour contrast penetrants it must mask out background contours and colours
- It must be easily and evenly applicable
- It must form a light and even coat
- There must be no fluorescing of the developer when used with fluorescent penetrant
- The penetrant bleeding from a discontinuity must easily wet the material
- When used with a colour contrast penetrant it must be of a highly contrasting colour. It seems agreed that the best colour is white
- It must be readily removable after the test is completed
- It must be non-toxic and non-irritant
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