Share 'As soon as a water-based product hits oil, it's going to separate. You can watch it happen in real time by adding some oil to a glass of water and seeing how they repel each other - that's what will happen on your face.' Instead of oil, Megan said you're better off prepping your skin with a hyaluronic acid or a hydrating primer. Her favourites include Napoleon Perdis' Auto Pilot Pre-Foundation Primer ($59), which helps to minimise redness and irritation while preparing the skin for makeup. Megan (pictured) said you need to steer clear of any oil-based products during the summer months, and ditch your long-wear full-coverage foundation