To answer your question, I think I need a little bit more information. Why are you using primer? What aspect of Laura Mercier Primer makes you love it so much?
I use Neutrogena SPF 50 sunblock the minute I get out of the shower. I let that sink in while I towel off, etc. Once that has been absorbed, I put on some foundation (Chanel Vitalumiere), conceal (undereye with revlon undereye concealer, rest of face with Laura Mercier Secret Camo), then I powder with Chanel Loose Powder. The loose powder makes my face nice and matte, and prevents face sweat. For some reason, my upper lip and forehead sweat profusely (attractive!). I found that this powder is basically the only thing that prevents this.
Then I do the rest of my makeup. But that's my base layer. For my purposes, I like foundation more than primer, and I think regular SPF works better than SPF mixed in with foundation, primer, etc.
If you like your primer because it prevents your makeup from sliding off, I'd skip the primer, use regular SPF, and add some loose powder to your base layer. If you like your primer because it evens out your skin, I'd use regular SPF and add in some foundation.
Here is a link to Lisa Eldridge's take on primer:
http://www.lisaeldridge.com/video/1670/primer/
Best of luck!