We show that a polymer-based route to ceramics can be implemented into additive manufacturing by reducing the time for pyrolysis to about a second, which we call flash pyrolysis. Repetitive deposition of nanometer scale coatings of the ceramic, in this way, is employed to create defect-free infiltrations of carbon fiber composites. The mechanical strength of the fibers is retained in the composite. Excellent wetting properties of the polymer precursor permits three-dimensional, conformal coating through the three stages of infiltration: nanoscale coating of the single fibers, filling of interstitial spaces between the fibers, and a buildup of the coating over the entire composite. The flash pyrolysis method will enable a new genre of polymer-derived ceramics made into net shape by this unusual method of additive manufacturing.