My 10 year old poly pad used for eventing in all weathers is kept sparkly white on a 40 deg wash with Ariel Liquitabs and a scoop of Vanish Oxy Action - totally magic stuff. Does wonders on ground in poo and sand stains on white boots as well!
White boots are the very reason I posted this! Mine have been used once but in a school that had grey sand. They now have nice grey sand marks on them!! I washed them with the Vanish Oxy Action in the wash but they are still there, so I'm currently soaking them in it in my sink - you wait til OH sees!!!
I must admit the insides of mines have been slightly marked by the grey fibre sand at Addington but any marks came off the outsides. Maybe try a slightly hotter wash?