It's tempting to do - add a bit of foundation to your favorite moisturiser to make it tinted, perhaps a bit of sunscreen to make it SPF & hey presto your own unique perfect potion. They're all got preservatives in, so what wrong with that?
Whenever you’ve got water in a product, you need a broad spectrum preservative to stop nasties growing. If bacteria or fungi grow within your skincare & you apply it to your skin this can cause irritation, rashes, blemishes & skin infections.
The concentration of a preservative is very important, so if you dilute it by mixing different products, you weaken it &/or render it completely ineffective.
"Ah" you say "But, what about if I make sure the % levels are about the same by adding similar products?"
Preservatives are chosen based on the composition of the product they are in- if you mix products together that blend might not work with the ingredients of your new wonder potion. This is especially true if one of those products contains "skin foods" like milk or herbal infusions (often found in "natural" products) - these really stretch a broad spectrum preservative to its limit.
The easiest way to think about preservatives is to think of cosmetics like you do food. Flour, salt, water & yeast last for ages in their pure state when stored properly. Combine them into a lovely bread bloomer & you will have something that will sprout mould in a matter of days.
You really shouldn't combine cosmetic products.🤔
"Well its never harmed me, or the YouTuber who told me to hack it, so I'm doing it"
OK. If that's your bag here's some tips to hopefully reduce your chance of problems:
1. Store the final product in the fridge - this will slow the rate of bacterial growth. Don't keep it in your hot steamy bathroom or on the windowsill.
2. Don't use your fingers - use an applicator & throw it away or clean it after each use or better find a bottle with a dispenser (clean & disinfect it before filling)
3. Take the use by date and take at least 6 months off it. If you're at that date, throw it away.
4. If it smells odd throw it away.