In rec.backcountry Bill Tuthill wrote:

Usually I wear clothing, which depending on fabric, works very well.
One day rafting I used SPF 40 instead, reapplying every time my legs
got wet, which was about every hour. Got a mild sunburn.

There is a product called 'Sun Guard' for increasing protection from the sun
through clothing. You put some in the washing machine when washing the
garment you wish to treat.