A fibrous polyamide or wool substrate having resistance to staining by acid colorants comprises, a fibrous polyamide or wool substrate having applied thereto a sulfonated aromatic condensation resin, said resin being a condensation product of a sulfonated dihydroxydiphenyl sulfone, a dihydroxydiphenyl sulfone, a sulfonated naphthalene and an aldehyde; or a sulfonated dihydroxydiphenyl sulfone, a sulfonated naphthalene and an aldehyde; optionally a methacrylic acid polymer is also applied thereto as well as a fluorochemical; an aqueous formulation of the resin is also provided which formulation may include the methacrylic acid polymer as well as the fluorochemical Additionally there may be employed a salt to enhance exhaustion of components onto the substrate; a method of treating a fibrous polyamide or wool substrate employing the formulation is also provided.