We use a sponge in the studio not only or wiping the piece down to prepare it for paint (don't worry we will prepare your pieces before you collect so you don't need to purchase a sponge) but it can also be used in the painting process.
We recommend you use a sponge if you wish to paint a base colour on a piece as painting is likely to leave brush marks (Paint 1st coat, once dry then sponge over it). Also if you want to create a sponged border or you can use to create a sponged effect on pieces. See examples above.
Any sponge would do so you may have a sponge knocking about at home you could use but please be careful there are no chemicals or detergent on the sponge as this could affect your piece and cause problems with glazing.
The underglaze paints used on pottery are water based so just run sponge under the tap while squeezing it until the paint has all come out.