[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. It takes trial and error to find this combination. There are no rules unless the product is a serum. I generally get positive results when I mix it with a facial oil.
When a product is really hard, like a balm or a thick oil, I mix it with a mist, serum or toner to lighten it and generate a microemulsion that feels lotion-like and in the skin sinks instead of occlusive to rest on it.
*** This really has nothing to do with the increase of the benefits or performance of a product – it has everything to do with the fact that it feels as I would like to have on my skin. ***
However, some people achieve the same effect through LAYERING when it comes to balm / oil: applying oil / balm to the skin that is damp with fog / serum / toner (first apply the water product, then the lipid) and some people It is better to apply lipid first and then massage the mist / serum / toner over it to achieve the same effect of "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 products you use.
There may be about 10 serums I want to use, but applying 10 serums to my skin makes it feel so sticky, coated. Then I emulsify a pump with 2-3 of them all and apply it as a layer. So I moved it from 10 to 3 layers and it feels a lot 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 make 1/10 of a pump of all 10 products in a giant microemulsion, then yes. Generally, mixing 2-3 serums / lotions of your skin provides maximum absorption and a longer lifespan 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 layered, you can sometimes microemulsify them together and then apply this mixture without pilling problems.
There are exceptions to this rule – the most important are peels. Peels should be applied undiluted to work as intended (which is why you should apply them first and then take a 5-10 minute break before applying other skin care products). 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 is now the right strength to desquamate your skin without causing irritation.
Carefully choose, wherewith shall it be diluted, 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 called mixed) is sun protection. You will only get the desired sun protection factor if you apply it generously at 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, otherwise there is a risk of irritation or excessive peeling.
Many think that the combination of vitamin C and niacinamide (B3) is also problematic. It is actually not. This is because niacinamide is converted to nicotinic acid in low pH solutions, with A SPECIFIC FORM of vitamin C (ascorbic acid) active. Nicotinic acid causes redness and reddening of the skin.
The combination of 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) is perfectly fine and there is no risk of nicotinic acid formation.
However, it turns out that the combination of ascorbic acid with niacinamide is ALSO FINE, especially if you layer or mix two different products. The reason for this is that the conditions for producing nicotinic acid are simply not met, namely prolonged exposure to low pH AND very high heat (how hot in the oven).
One case where layering is preferred is when the skin is really dry and you try to pump in and trap moisture. In this case, layering is key, starting with moisturizing serums and lotions and ending with a 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 like to create their own ideas and experiment 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 shift workers.
What about you – do you layer or mix? Does it depend on the products, your mood, other factors?