Maintaining polyester American flags is easy.
Polyester, other synthetic fabrics are generally safe to wash in a machine. Set the washing machine on the gentle cycle with warm water. Add a mild liquid laundry detergent to the wash water. You can also hand wash the flags with a mild liquid detergent made for delicate items.
Polyester is faster drying, easier to dye and abrasion resistant. Polyester fabric also tends to be either moisture trapping, or moisture wicking, neither of which are ideal for cooling in warm weather.