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Use a natural sea sponge (sponges harvested from the sea) for an interesting, multi-layered effect, similar to the look of granite (but with any color you choose). You can find natural sponges for around $10 (£7.10) at a hardware store.  Natural sea sponges often come in sizes of 5–6 in (13–15 cm). You can use a kitchen sponge, but it won't look quite the same. Choose the colors that you want to paint with and pour them into separate disposable bowls. You can use foam bowls, paper bowls, or regular bowls that you've set aside for craft use (just remember they'll be dirty afterwards). Run water over the sponge enough to get it damp, then crush it with your hands until the water no longer runs out. If you don't wring the sponge out, your paint may come out too watery and not adhere to the foam well. Choosing the paint you want to use, dip the sponge into it, blotting a little bit of paint off the sponge onto the newspaper so that you don’t make drips on your foam. Try practicing your sponge painting on the newspaper before moving to the foam; this will give you a better idea of what you want to do on your project. Touch the sponge to the foam gently, letting it rest on the foam for a few seconds before pulling it away. Repeat until the entire project is covered in paint.  Try rotating the sponge while painting to get different effects. Dampen the sponge with clean water and wring it out before choosing another color. It's important to let the paint dry before adding another coat, or it may end up looking uneven. You can wait up to 60 minutes to ensure the paint dries fully. Use the same technique to paint another coat. Focus on spots where the paint has seeped in and left white spots. Aim to create an even layer so that all the foam is covered. Give your foam project at least an hour to dry before picking it up. Wash your sponge in clean water and wring it out thoroughly, then let it dry in a ventilated area. Be sure to wash your hands with soap and warm water after handling paints.

Summary:
Use natural sponges for an artsy, granite-like effect. Put acrylic, water-based paints in separate disposable bowls. Dampen a sponge with water and wring it out. Dip the sponge into the paint, then blot some onto the newspaper. Paint by applying the sponge softly to the foam’s surface. Wait at least 15 minutes for the paint to dry. Apply another coat of paint. Let your project dry for an hour and wash your sponge.