Volume for Flat Hair
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A: The reason for the "time limit" or "waiting period" following most chemical services on the hair is to allow the hair time to "re-normalize" before subjecting it to treatment that could cause potential damage.
A better suggestion would be to wait the period of time instructed and dry your hair with your head inverted in order to generate a maximum of root lift and volume in the style, without losing the smoothness of the cuticle of the hair.
Refraining from using the flat iron is also a good way to keep from flattening the hair too much. You could, however, use the flat iron only on the ends of the hair if you need a smooth finish on the ends without flattening the scalp area too much.
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See also:
Hair straightening
Blow drying basics