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You will probably gag, but it's worth it! Work the vinegar through your hair slowly, over a 10 minute period to get maximum saturation. Refrain from rinsing. This gives the vinegar time to sink into the hair and scalp. Use a treatment with a  2%  salicylic acid concentration. Again, pour slowly and allow liquid to soak into your hair. Leave the vinegar and acne treatment in your hair for about 30 minutes. Do not rinse the vinegar and acid out of your hair before doing this. Scrub this onto your scalp and hair. Let this mixture sit for about 20-30 minutes. If possible, concentrate these products on more the back of your head. Samples are most commonly taken from this area. Do not wash or condition. Rinse well. Use the conditioner treatment that usually comes with the hair colour kit. Instructions for this remedy vary - some recommend repeating once a day in the four or five days leading up to your test, while some recommend only one treatment.

Summary:
At home before your test, begin by saturating your hair with white vinegar. Let the vinegar sit for 15-20 minutes. Next, soak your hair with a salicylic acid acne treatment. Work one capful of liquid detergent through your hair thoroughly. Add a dash of water to 1 scoop of powder detergent to make a paste. Rinse all the products out of your hair. Dye your hair using a standard store-bought dying kit. Repeat as needed.