Can I use a windshield rain repellent on a motorcycle helmet visor or plastic windscreen?

It is not recommended. Car windshield treatments are formulated for glass, while helmet visors and most motorcycle windscreens are made of polycarbonate or acrylic, which can react differently and may have anti-fog or scratch-resistant coatings that solvents can damage. Use a water repellent made specifically for visors and plastic screens, and never apply a repellent to the inside of a visor with an anti-fog insert. Test any product on a small area first.