I hear this question a lot: What's the difference between a foundation primer and an eyeshadow primer, do I need an eyeshadow primer and how do I use it?
What's the difference? Basically, most of the top foundation primers out there (think Smashbox Photofinish) have a silicone base--so they're a bit more slick and smoothing, and they tend to create a "floating" effect on your skin so foundation won't settle where you don't want it to. When it comes to your eyes, you're looking for a finish that will help the eyeshadow cling and stay put for as long as possible, ideally without creasing.
Eyeshadow primer may still have some silicone-based ingredients, but it's usually creamier in consistency. Think of foundation primer as regular paint primer that's used to cover flaws first and foremost; think of eyeshadow primer as a paint primer you'd use if you wanted to paint over a non-adhering surface (eg, a mirror)--it helps the paint to bond or adhere to the surface.
Do you need an eyeshadow primer? Most likely, no. For the most part, you can use a bit of your foundation just as easily, so why waste the money on a separate product? The key here is to use just a bit--eyeshadow primers typically have a lighter feel than foundations. If you foundation is a heavier cream--which can be the case if you've got over 40 skin, you may want to create a lighter feel first by mixing it with a small drop of light, sheer or oil-free moisturizer. You want to avoid a greasy, slick feel.
How do you use an eyeshadow primer?
Simply apply a small amount to your upper eyelid and blend it up and out toward your brow. Make sure you get as close to your lash line as possible to ensure even application of your eyeshadow. It should look like a light, creamy wash of even color over your eyelid. Let it set for a minute or two, then apply your eyeshadow as you normally would.
Should you feel like splurging on a separate eyeshadow primer--don't go for broke. A drugstore one to try: Lumene Beauty Base Eyeshadow Primer. It's available at most CVS stores.