[Disclaimer: this is hardly comprehensive or correct, but these are just my personal rules/preferences:]
In general, I mix if I want to change the skin feel of a product OR if the skin feel of the products in layers is too heavy .
If a product is particularly sticky, I mix it with a product that reduces or removes the stickiness. Experiments are required to find this combination. There are no rules, unless the product is a serum I can generally get positive results when I mix it with a facial oil.
If a product is really heavy, like a balm or a thick oil, I mix it with a mist, serum or toner to lighten it and create a microemulsion that feels lotional and sinks into the skin instead of occlusively to rest on it.
*** This really has nothing to do with increasing the utility or performance of a product – it has everything to do with how it feels on my skin. ***
However, some people achieve the same effect through LAYERING when it comes to a balm / oil: apply oil / balm with mist / serum / toner to damp skin (first apply the water product, then the lipid) and some people Prefer to apply the lipid FIRST and then massage the mist / serum / toner over it to achieve the same effect as "lotioning" the oil / balm.
When it comes to layering or mixing, experiment to find out what works best for you – it may vary depending on the product you use.
There may be 10 serums I want to use, but when I put 10 serums on my skin it creates a sticky feeling. Then I emulsify a pump of 2-3 pieces and apply it as a layer. So I moved it from 10 to 3 layers and it feels much more breathable and lighter, but I still got all the benefits of the 10 products.
Does this dilute the products? Not as much as you think – if you tried to pump 1/10 of all 10 products in a giant microemulsion, then yes. In general, mixing 2-3 serums / lotions ensures maximum absorption of your skin and a longer shelf life for your products. In general, we tend to overuse in skin care, especially for products that contain active ingredients.
A nice little trick is that if two products are on top of each other, you can sometimes microemulsify them together and then apply this mixture without pilling problems.
There are exceptions to this rule – the main rule is peelings. Peels should be applied undiluted to work as intended (so you should apply them first and then take a 5-10 minute break before applying any other skin care product). But – you can use this exception to your advantage – if a peel is too strong for you, you can dilute it and check if it's now the right strength to desquamate your skin without irritation.
Choose carefully what to dilute it with, choose a simple serum or oil without many acids or active ingredients 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 (also known as mixed) is sun protection. You will only get the desired sun protection factor if you apply it generously in full strength!
There are some ingredients that you shouldn't mix – two that come to mind immediately are retinol and acids. In general, you should take care never to layer or combine two peeling products or ingredients, as this can cause irritation or over-peeling.
Many consider the combination of vitamin C and niacinamide (B3) to be problematic. It is actually not. This is because niacinamide is converted to nicotinic acid in low pH solutions, which must be considered active in a specific form of vitamin C (ascorbic acid). Nicotinic acid causes redness and reddening of the skin.
It is perfectly fine to combine B3 and any form of vitamin C that is pH independent (and there is a ton, we use two called sodium ascorbyl phosphate and tetrahexyl decyl ascorbate) and there is no risk of the formation of nicotinic acid.
But it turns out that the combination of ascorbic acid with niacinamide is ALSO FINE, especially if you layer or mix two different products. This is because the conditions for the production of nicotinic acid are simply not met, namely LONGER exposure to low pH AND very high heat (like hot in the oven).
Layering is preferred when the skin is really dry and you are trying to pump in and lock in moisture. In this case, layering is crucial, from moisturizing serums and lotions to silicone, oil or balm to prevent transepidermal water loss (TEWL).
So which is better – mixing or layering? Honestly, I think it depends on the personality type!
Frugal people, people who enjoy simplicity or efficiency, and creative people who enjoy creating their own creations and experimenting a lot are usually mixers.
Skin care lovers, people who perform an extravagant self-care ritual, people who prefer to follow skin care instructions for a T or similar structure are usually layers.
What about you – do you layer or mix? Does it depend on the products, your mood, other factors?