With each client, whether you are applying a full set of lashes or just a fill, you should always cleanse the lashes prior to starting the service. Any debris or residue on the natural lashes will cause a barrier between the natural lash and the adhesive.
Even if the client has visibly clean lashes, use any recommended method of pretreatment to ensure the lashes are "squeaky clean"; most often, you will find that there is still some makeup residue or natural oils still on the lashes at the lash line.
Educate your clients to wash their lashes daily using a special brush and lash foam or any oil and alcohol-free cleanser. Baby shampoo is a great alternative - it's gentler on the eyes. It's also important to avoid sharing makeup with others to avoid cross contamination.
By washing lashes on a regular basis your clients save their own time during the appointment and avoid eye diseases!