Hairdirect and Prohairlabs teach that layers have to be thin and even. What happens, if I do them thick? I think the bond maybe be stronger, if they are thicker. Because of more glue?
thick layers won't dry as fast or as well because the bottom of the glue will not be able to cure so well. The bond will be weaker.
That's why they recommend multiple thin layers, applying each later a few minutes apart.
