Hair Strengthener Retrò is a specific treatment for dry, weak and dehydrated hair. Silk protein help strengthen hair damaged by chemical treatments (dyed, tinted or permed hair) the harmful effects of air pollution (smog, wind, salty breezes) or by prolonged exposure to the sun. Wash your hair with the specific Retrò shampoo and rinse. Put a small quantily of shampoo on your hair and comb it in all the way to the very end. Let your hair absorb the shampoo for 5 minutes and rinse again.