If you're wondering whether you should put protein filler on wet or dry hair, the answer is both! Depending on your hair type, you may find that one works better for you than the other. If your hair is particularly dry or damaged, you may want to try using protein filler on wet hair first. This will help to hydrate and condition your strands while also providing a boost of protein. If your hair is more oily or prone to build-up, however, you may find that using protein filler on dry hair works better. This will help to absorb any excess oil and prevent buildup from occurring. Ultimately, it's up to you to experiment and see what works best for your own hair type!