I typically view foundation primers more as a nice-to-have kind of product, but for oily and many combination skin types, they can often be a necessity. These are the two I rely on to combat my oily t-zone and help my foundation last throughout the day.
Laura Mercier Oil Free Foundation Primer and Smashbox Photo Finish Foundation Primer
Laura Mercier claims: "Perfect for all skin types. Laura Mercier Oil Free Foundation Primer creates a smooth and invisible layer to prepare and protect the skin. Your foundation will stay fresh all day."
The Laura Mercier Oil Free Foundation Primer has the consistency of a lotion and smells like sunscreen (in a bad way, not the tropical coconut kind of way). It absorbs quickly into the skin and does not leave behind any residue. I don't notice that my foundation applies any more smoothly on top of it, but I do find that it lasts about 3 or so hours longer on my skin than it would without primer before starting to show signs of breaking down due to the oils on my face. It also extends the time to my first blot, as well as prolongs the time between blots.
Smashbox claims: "Increases the longevity of foundation while reducing the appearance of fine lines and pores."
The original Smashbox Photo Finish Foundation Primer is an oil-free, clear gel that has a powder/velvet-like finish due to the silicone content. It also smells a bit funny, but I can't quite put my finger on it. Silicone does not tend to irritate my skin (I know it can for others), and I love that it fills in my pores and makes my skin look and feel ridiculously smooth. My foundation applies noticeably more smoothly over it. I find that my foundation lasts much longer on the skin, often not showing any signs of breakdown at all, even by the end of the night when I take my makeup off. It also does a fantastic job at keeping the oil on my t-zone at bay for several hours, and I tend to need to blot even less when I use this primer than I do when I use the Laura Mercier one.
Overall, I much prefer the Smashbox primer to the Laura Mercier one for my particular needs, although those who are sensitive to silicones may want to steer clear. I love how smooth my foundation looks and feels on top of it, and I was impressed by how much longer my foundation stays shine-free and fresh when I use it. Both Laura Mercier and Smashbox make several different primers for various purposes and skin types, although I think I've found my match in the original Smashbox Photo Finish Foundation Primer.
The Laura Mercier primer retails for $32 for 1.7 oz., while the Smashbox primer retails for $36 for 1 oz.
Do you find primer to be a necessity? Which one do you use?