substrates not inhibiting polymerization include wood wood by products such as paper p aramid fibers molded polyimide parts porous polyurethane and leather whether a substrate will inhibit polymerization must be determined empirically substrate by substrate and may vary for the same substrate depending upon prior finishing and treatment the present invention also provides a fluoropolymer substrate composition wherein the substrates are open structures with interconnecting pores throughout their bulk and the level of fluoropolymer in the fluoropolymer substrate composition is about 0 1 to about 300 of the weight of the substrate substrates useful in this invention include wood paper leather porous polyurethane and aramids and polyimides that have been precipitated as porous particulates or porous fibers and then left wet dried or molded only so far as to preserve enough porosity for subsequent penetration by fluoromonomer and initiator preferred substrates are porous aramid polyimide particulates and polyimide parts when a preferred substrate is used the porous aramid or polyimide is immersed for about 1 minute in a 0 1 to 0 2 m solution of hexafluoropropylene oxide dimer peroxide dp 1 in cf 3 cfhcfhcf 2 cf 3 solvent the excess solvent is filtered off or is drained from the aramid or polyimide and the still damp polymer placed in a container with 1 atmosphere pressure of tetrafluoroethylene gas until the substrate has gained preferably 5 to 20 of its weight by polymerization of the tetrafluoroethylene to poly tetrafluoroethylene the preferred aramids are poly p phenylene terephthalamide hereinafter u201c ppd t u201d fibers and poly m phenylene isophthalamide hereinafter u201c mpd i u201d in the form of fiber particles pulp or fibrids that are.