Hi,

I am thinking of layering Exfol serum under Lactic 10.

Does it mean that I will get 12.5% total acid, or it doesn't work that way?

Because of the increased volume, if you use equal amounts of each product, you will get one-half of 12.5% or 6.25%.

I see. What if I were to apply Exfol and let it dry/absorb first? Would that make a difference?

Yes, to some extent it would increase the product's strength.