Don't let yesterday's rainstorm dampen your plans for a picnic. You can waterproof a cotton or wool blanket by adding a protective backing of water-resistant fabric such as ripstop nylon or oilcloth, and safely lounge on damp grass.
Waterproof Blanket How-To
1. Cut fabric to the exact dimensions of the blanket, adding 1/2 inch on all sides for hem. Fold edges of backing under 1/2 inch, and place 2-inch-wide twill tape on top; machine-stitch both inside and outside edges of tape around entire perimeter, mitering tape at corners.
2. With a hammer or mallet, fix snaps along edges of the backing and the blanket (snap kits with instructions are available at sewing stores): Center a snap in the twill tape at each corner, then attach snaps at intervals of 10 to 12 inches around the whole border. Attach the "male" half of snaps to the backing first; then mark the placement of the "female" half of snaps carefully on blanket for a good fit, and attach snaps. The backing may be detached when it is not needed.