There are a number of ways to avoid this, when you use a new file rub the sharp edges off it with another file as this is the most common reason for cutting a client. Also use your own fingers to protect the surrounding skin of your clients when filing enhancements. If you do cut a client remember to throw that file away. Blood spots may still be present on the file and you may not be able to see them with the naked eye, you need to protect the health of your other clients.
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You will need to be trained so maybe first go on a nail course, Sally do them or even CND for a good course. If you protect the clients skin with your fingers and make sure the edge of the file is not sharp you will be fine. Just be careful and go slow!
Practise Practise Practise I have been a nail tech for 20 years I have never cut a client . Hole the nail with the skin pulled back and as the others stated file the edges of a new file. If you do cut someone you should sterilise your file which should be done in any case .... if you throw all of your files away after use this will become expensive. do not use acetone to blend a tip however you can purchase tip blender