abrasion in order to lend itself to handling and transportation operations and one can employ for this purpose epoxide resins prepared from a combination of epoxy resin condensate of epichlorhydrin and bisphenol a and a hardener with a base of accelerated or substituted dicyandiamide or cyclic amines polyanhydrides polyisocyanates etc such resins have however a mediocre resistance to the elements and a preferred form of implementation of the invention utilizes powdered polyester resins as a matter of fact the tests carried out by the applicant have proven that the best results are obtained with external layers of polyurethane composed of a polyester resin with free hydroxyl functions and a hardener of the blocked polyisocyanate type as saturated polyester resins one will be able to employ resins resulting from the polycondensation of aromatic or aliphatic polyacids and of di or tri functional polyols as examples of aliphatic polyacids one can cite adipic acid azelaic acid and sebacic acid as examples of aromatic polyacids one can cite the orthophthalic isophthalic terephthalic acids or their esters making it possible to make transesterifications as well as their halogenated or hydrogenated derivitives as examples of polyols one can mention ethylene glycol propylene glycol neopentylglycol trimethylolethane trimethylol hexane pentaerythritol 1 1 isopropylidene paraphenyleneoxy di ethanol or dipropanol and in a general manner the polyols whose molecular weights are between 500 and 5000 but preferably between 1000 and 3000 the hydroxyl index will preferably be between 30 and 300 and preferably between 30 and 100 as it has been indicated above crosslinking of these polyester resins can be accomplished using blocked polyisocyanates this type of polyester resin can equally be utilized with other hardeners etherified melamine essentially hexamethoxymethylmelamine or anhydrides of.