Don't waterproof your picnic blanket. Get a tarp.

Don’t. Just buy a cheap polyethylene tarp.

Waterproofing spray or wax on a fabric blanket won’t hold up against wet sand. Sand is abrasive, and constant folding and shaking will wear the coating off fast. Plus the blanket will stay damp underneath anyway because moisture wicks through the fabric where you sit.

A tarp (the blue or silver kind from the hardware store) is what you actually want. It’s waterproof, sand shakes right off, and it weighs nothing. Throw a thin blanket on top if you want softness. When you’re done, shake it out and fold it up.

If you really want one blanket to do everything, buy a dedicated waterproof picnic blanket with a bonded backing. They’re about $30 and last years.

This is not where you reinvent the wheel.