For trowelling fine-grained plasters, it is best to use soft, thick sponges, such as those used by tile layers for pointing. You should buy a few of those sponges, as the best trowelling results are achieved when the sponge is dry and clean. When the sponge is dirty and its pores are filled with plaster particles, it should be washed and left to dry, and the works should be continued with the use of another sponge.