Oil contamination can leave fisheyes, stains, gloss variation, smoke, poor paint adhesion, or weak secondary bonding. The source may be hydraulic leakage, pneumatic lubricators, mold release, handling equipment, or maintenance work.
Heating can spread a small deposit over a larger area or drive it into a textured surface. Wiping after forming may remove the visible film while leaving a surface that still fails coating or bonding.
Diagnosis and correction
The contamination pattern should be traced upstream. Repeated marks in one machine position suggest a leak or tool source. Random fingerprints or edge stains point toward handling. The chemical identity should be established before a cleaner is selected because aggressive solvents can craze stressed plastics.
The leak or application practice must be corrected, followed by validated cleaning of the sheet, tool, and downstream fixtures. Release agents should be applied only where required and in a controlled amount. A clean appearance is not sufficient when the part will be painted, bonded, or printed.