I am creating monoliths from a hard carbon powder mixed with small amounts of PFA. After sintering, an orange-brown residue coats the pellets. As PFA is soluble in acetone, I tried sonicating the pellets in acetone and again with distilled water. It is a very sticky residue, and I am also unable to brush it off. Any suggestions (aside from mechanical polishing) to remove a hydrocarbon residue?