Nikwax say - For waterproofing rain gear.

Easy to use wash-in waterproofing for all breathable waterproof clothing

Adds water repellency, revives breathability and reduces water absorbency without tumble drying

Enhances the performance of your gear, keeping you warmer and drier

Recommended for Gore-Tex®, Sympatex®, eVent®, Nikwax Analogy® and all other waterproof fabrics