I have used oxyclean with excellant results. vacuum the area to remove any dry powder stains prior to treatment. Mix oxyclean in a spray bottle, spray it on, dab it- but do not rub it as rubbing will effect the nap of the fabric and it may be visible after it dries. Let it dry. repeat as necessary.. If you start at the outside of the edge, the stain will not spread.
I hope this helps.
95 Newmar Kountry Star, Spartan Chassis, Venice, Fla