[Disclaimer: this is hardly comprehensive or correct, but these are just my personal rules/preferences:]
In general, I mix when I want to alter the skinfeel of a product OR when the skinfeel of the products when layered is too heavy.
If a product is particularly sticky, then I’ll mix it with a product that lowers or disappears the stickiness. It takes trial and error to find this combination, there are no rules, except if the product is a serum, I generally get positive results by mixing it with a facial oil.
If a product is really heavy, like a balm or a thick oil, I’ll mix it with a mist, serum or toner to lighten it up and create a micro-emulsion that feels lotion-y and sinks into the skin instead of resting on it occlusively.
***This really has nothing to do with increasing the benefits or performance of either product – it has everything to do with making it feel the way I want it to feel on my skin.***
However, some people achieve the same effect by LAYERING when it comes to a balm/oil: applying oil/balm onto skin damp with mist/serum/toner (so applying the hydric product FIRST, then the lipid) and some people prefer to apply lipid FIRST, and then massage the mist/serum/toner over it to create the same effect of “lotionifying” the oil/balm.
When it comes to layering vs mixing, do some experimenting to see what works best for you – it can vary with the products used.
Maybe there are like 10 serums I want to use, but layering 10 serums on my skin results in such a sticky, coated feeling. So then I’ll micro-emulsify a pump of 2-3 of them all together and apply that as one layer. So then I’ve taken it from 10 layers to 3 layers and it feels a lot more breathable and lighter but I’ve still gotten all the benefits of the 10 products.
Does this dilute the products? Not as much as you think – if you tried to do 1/10th of a pump of all 10 products in one huge micro-emulsion, then yes. But in general, mixing 2-3 serums/lotions gives your skin basically maximum absorption and helps your products last longer. In general, with skincare, we tend to over-apply, especially products that contain actives.
A neat little trick is that if two products pill when layered, sometimes you can micro-emulsify them together and then apply that mixture without any pilling issues.
There are exceptions to this rule – the main one is exfoliants. Exfoliants should be applied undiluted to work as the formulator intended (which is why you should apply them first and then take a 5-10 minute break before applying other skincare). But – you can use this exception to your benefit – if an exfoliant is too strong for you, you can dilute it and see if it now is the right strength to desquamate your skin without causing irritation.
Be careful choosing what to dilute it with, choose a simple serum or oil without a lot of acids or actives and with a neutral pH (5.5-6.5). Two good options are a pure hyaluronic acid serum or a pure carrier oil.
Another key product that should never be diluted (aka mixed) is a sunblock. It only gets you the desired SPF if applied at full strength liberally!
There are some ingredients you should be careful to not mix – two that come to mind immediately are retinol and acids. In general, you should be careful to never layer or combine two exfoliating products or ingredients because you will run the risk of irritation or over-exfoliation.
Many think that combining Vitamin C and Niacinamide (B3) is also problematic. It actually isn’t. The reasoning is that Niacinamide converts to Nicotinic Acid in low pH solutions, which ONE SPECIFIC FORM of Vitamin C (Ascorbic Acid) needs to be active. Nicotinic Acid causes redness and flushing in the skin.
Combining B3 and any form of Vitamin C which is pH independent (and there are a ton, we use two called Sodium Ascorbyl Phosphate and Tetrahexyldecyl Ascorbate) is totally ok and there is no risk of Nicotinic Acid formation whatsoever.
But, it turns out combining Ascorbic Acid with Niacinamide IS ALSO FINE, especially if you are layering or mixing two different products. The reason for this is that the conditions simply are not met for the creation of Nicotinic Acid, namely PROLONGED exposure to low pH AND very high heat (like oven hot).
A case where layering is preferred is when the skin is really dry and you are trying to pump in and lock in moisture. In this case, layering is key, starting with hydrating serums and lotions and ending with a silicone, oil or balm to prevent trans-epidermal water loss (TEWL).
So, which is better – mixing or layering? Honestly, I think it comes down to personality type!
Frugal people, people who enjoy simplicity or efficiency, and creative people who enjoy creating their own concoctions and doing a lot of experimenting, are usually mixers.
Skincare aficionados, people who are doing an extra extravagant self-care ritual, people who prefer to follow skincare instructions to a T or like structure, are usually layerers.
What about you – do you layer or mix? Does it depend on the products, your mood, other factors?