How paint this part?

depending on the size , i would just make a stencil of the longest diagonal section and then run that along the entire length of the segment, maybe that wil work
Jeez thats a big fitting if its a metre long LOLOL..Mate, theres probably a dozen things that clould do that in almost one hit..Try finding some diamond shape fencing wire or mesh, at a meter wide I'm sure some finer fencing square mesh would do it..Then cut it to suit the shape and you could even shape it further to suit perspective by squashing the ends down a bit..Spray a light silver base to suit the groove colour, lay the wire down and then add some shading to suit the raised edges, after that initial spray you may even then be able to offset the wire a bit and respray so it has that doubled look like when painting carbon fibre look..Other material like fishnets etc cld be used as well but not being stiff they are a bit harder to work with..It really does I suppose depend if ya can find a wire meshing to suit that size, the only real alternative if ya want it pretty clean is a heckload of taping with some pinstripping tape..GL