Remove any water from the iron. Preheat the iron to low heat.
Slide the bathing suit around the nose of an ironing board with the plastic stain facing up and the opposite side of the suit sitting under the board.
Tear or cut a brown paper bag into several pieces large enough to accommodate the iron's soleplate.
Place one piece of brown paper over the plastic stain. Set the hot iron on top of the brown paper.
Move the iron back and forth over the stain. Continuously look under the iron or the paper to check for melted plastic starting to saturate the paper. Once plastic is seen absorbing into the paper, remove the paper.
Scrape away any excess softened plastic, using the edge of a spoon. Scrape gently to avoid spreading the plastic or embossing the fabric.
Cover the stained area with a new piece of brown paper. Iron the paper, as before, and remove it once plastic saturates it. Continue applying and melting plastic to the paper, until little or no evidence of the plastic remains.