J. Jason Wendel, MD, FACS says:The rule of thumb when applying skin care products is THINNEST to THICKEST. Depending on your glycolic acid (is it serum based, pad, etc?) If it is a pad- I would do glycolic pad, vitamin c, TNS, and then HA5 (then sunscreen of course). But again- thinnest to thickest is the best way to go...