Wella Professionals Resist Strengthening Serum for Vulnerable Hair. Wella Professionals Resist Strengthening Serum for Vulnerable Hair is an intensive treatment for mature and vulnerable hair types. Enriched with extracts of Rooibos and Karite, this strengthening complex works to deeply penetrate hair, with a restoring and reviving effect for a healthy finish. Hair is left nourished and hydrated, with a renewed vitality and radiance.