Don't know if this is possible due to access issues but if you could use 000 steel wool to "sand" (wool?) the paint off.  This will leave fine scratches in the plastic.  Then air brush a light coat of Sig Lite-Coat over the area you just wooled and the scratches will disappear.  Should not have a compatibility problem if the first coat is misted.  Try on a piece of scrap canopy first so you know what to expect.  If the Rusteolum has not reacted with and softened up the canopy plastic it should work.