Milk of Magnesia has been touted, recently, as a good face primer. Stop. A main ingredient in Milk of Magnesia is Sodium Hypochlorite which is a standard household bleach. It may be in a very low concentration, but any sort of bleach is not really the best thing for skin. That said, as it acts as a de-greaser, you can understand why people recommend this as an option for a primer. (The main ingredient is Magnesium Hydroxide - an inorganic compound patented in 1818 for digestive issues; in other words, it helps you poo)
True, Sodium Hypochlorite does minimise oiliness and no doubt it has the potential to prolong your makeup wear. BUT it has a pH of 10.5. For comparison, ammonia has a pH of 11. Baking soda has a pH of 8.3. In order to control bacteria, your skin needs to maintain a mildly acidic pH. P. acnes, for example, is best inhibited by a pH between 4.2 and 5.6. Your skin has evolved to maintain this balance very effectively. Your sebaceous glands secrete a thin layer of acidic film called the acid mantle, which protects you from viruses, bacteria, and other potential threats.
Milk of Magnesia was formulated to neutralise stomach acid. This totally destroys your acid mantle, leaving your skin unprotected. Using Milk of Magnesia as a primer sounds like the perfect way to turn your face into a bacteria's dream home. So, when you use Milk of Magnesia as an everyday primer, it has the potential to be quite damaging. Milk of Magnesia may be effective for oil control, but there are lots of skincare primers that can help control oil that are actually supposed to go on your skin. Please, buy one of them, instead.