If your foundation isn’t sitting right on the skin, it could be caused by incompatible products coming together. What makes products incompatible is their base ingredients. Some products are water-based, while others are silicone- or oil-based.
If your primer is made with a base ingredient that doesn’t mix well with the base in your foundation, you’ll likely experience pilling. This sounds sciencey, but let us break it down.
Cosmetic chemist Javon Ford recently summarized how to navigate this tricky situation via TikTok. “While you can layer oil and silicone on top of water, you can’t do the reverse,” Ford explains. Think of it like painting, he says, similar to how you can’t add latex to an oil-based canvas.
You can also think about it in basic skin care layering protocol: This is why you first apply serums or essences (water-based) under creams or oils (silicone- or lipid-based). This is because water has the ability to sink into skin, while oil cannot. So oil (being a larger molecule) will sit on the surface, and not allow any water that you apply on top to penetrate.
But back to makeup. See, when you’re using incompatible products in the wrong order, it will become a goopy mess. As Ford explains in the comments, “If you put a water-based foundation on top of silicone primer, it can bead or flake off.”
So if your primer and foundation have different bases, just be sure to avoid putting a water-based foundation on top of a silicone-based primer. A water-based primer under a silicone foundation, however, is A-OK. And a silicone-based foundation with a silicone-based primer is also good to go; same goes for a water-based primer and water-based foundation.