Which of the following can cause false indications during penetrant testing?

Lint or dirt can indeed cause false indications during penetrant testing because these contaminants may mask or obscure flaws in the tested material. Penetrant testing relies on the ability of a liquid penetrant to seep into surface-breaking defects. If lint or dirt is present on the surface, it can create a barrier that prevents the penetrant from properly entering the defect, thereby leading to a false reading or a failure to indicate a flaw that is actually present. This highlights the critical importance of preparing the surface thoroughly before applying penetrant, to ensure that any existing defects can be accurately detected.

Other factors like heat applied to the sample can affect the testing process, but they do not directly obscure flaws in the same way that dirt or lint does. Overexposure to the penetrant may lead to excessive background fluorescence, but it does not necessarily result in the masking of defects themselves. Excessive air pressure can lead to issues in the application of the testing materials, but similar to overexposure, it doesn't directly cause false indications in the way that surface contaminants do.