How long should i leave on really old rust stain on concrete before rinsing off?

Apply full-strength using a garden sprayer. Allow stain to disappear and rinse. Repeat if necessary. Every surface is different, so always apply on an inconspicuous area before using. Will etch glass if not rinsed immediately. Spray surrounding plants and grass with water before and after use. Do not apply to any paint, stain or surface containing iron oxide pigments.

Apply full-strength using a garden sprayer. Allow stain to disappear and rinse. Repeat if necessary. Every surface is different, so always apply on an inconspicuous area before using. Will etch glass if not rinsed immediately. Spray surrounding plants and grass with water before and after use. Do not apply to any paint, stain or surface containing iron oxide pigments.