Sometimes the standard advice to wash with gentle detergent and perhaps some fabric
softener just won't cut it, and pressure washing
may be required.
Here's a simple tool that makes it easier.
Assemble a "TEE" fitting with plastic pipe.
The horizontal 'top' of the T will be just large enough to easily draw the rope
The vertical 'stem' of the T will be fitted with a garden hose adaptor (perhaps /w a shut-off valve).
Attach the garden hose to the adaptor, and /w water
running, pull rope, back & forth, through 'top' tee sleeve.