One of the crafts we offer the kids at the children's camp where I'm in charge of the craft program is sponge stenciling. I'd never thought of it, but it should work fine for clouds. I don't think you would need sea sponges, any sponge cut to a cloud shape will work. The trick to keep from making a mess is that you don't use a can of paint and then try to use the sponge like a brush or to squeese the paint on. You put a thin layer of paint in an upside down margerine tub lid or a coffee can lid. You want to use standard house paint, not thin watery paint ready for your air brush. Use the sponge like a stamp and the lid with paint like a stamp pad. Just dab the sponge on the paint lid and then dab it on the backdrop. Don't let the sponge sit in the paint and soak up an excess amount and don't try to "mush" it on the backdrop. If you are squeesing the sponge enough to distort it at all you are using too much pressure.