If you want to skip the primer and go straight to foundation, then you can just apply two coats of your liquid foundation to build coverage and help your foundation last longer.
If you use mineral powder foundation and want great coverage that lasts all day, then you can also apply one thin coat of liquid foundation to your face, then dust on your powder foundation.
However, if you want the advantages of a facial primer without the extra coverage, then it is best to stick with a traditional primer.
An exception is if you want to use your foundation as a lip or eye primer. Foundation can actually work very well as an eye shadow primer. While you can use the foundation alone as an eye shadow base to help your shadow last longer, Cara of the popular beauty blog Maskara.com uses it as part of a recipe to create her own DIY eye shadow primer.
According to Cara, she mixes 1/2 teaspoon of natural lip balm, 1 teaspoon corn starch, and 1 1/2 teaspoons of liquid foundation.She then applies the mixture to her eyelids before she applies shadow to help it last longer.