station 12 which is spaced from reel 10 and at which the surface of filament 11 is prepared to receive a subsequent glass forming coating when the core filament is carbon or graphite prepared as disclosed in previously mentioned u s pat no 4 376 803 the core filament will have a polyvinyl alcohol coating stage or station 12 in that case constitutes a furnace in which this coating is burned off in an air atmosphere alternatively and for fluorocarbon or carbonaceous fibers in general station 12 may provide a plasma etch using oxygen gas or any other reactive plasma medium for the surface treatment as the core filament moves through the station additionally for surface preparation of halide glasses gaseous treatment utilizing steam heated hydrogen and other reactive gases can be used for refractory oxide filaments hot halide gases and solutions such as hcl hf hi and other reactive gases and solutions can be sequentially applied to the filament surface for surface preparation after the surface treatment at station 12 the treated core filament enters station 13 which is spaced from station 12 a glass forming compound 14 as shown in fig2 b is continuously coated onto the outer surface of the core filament as it moves.