Cast magnesium and aluminum normally have a considerable amount of microshrinkage and gas porosity. These discontinuities are usually no problem when penetrant testing the raw casting; however, after machining into these internal voids, it is often very difficult to detect cracks because of the large amount of background. The best results will be obtained by using a:

high viscosity, high contact angle penetrant: fogged dry developer.

small meniscus spot penetrant; part dipped in wet developer.

visible penetrant and a heavy coat of developer.

Group IV water washable penetrant; a wet developer with a long drying time.