We present new mechanisms by which the isotopic compositions of X-type grains of presolar SiC are altered by reverse shocks in Type II supernovae. We address three epochs of reverse shocks: pressure wave from the H envelope near t ¼ 106 s, reverse shock from the presupernova wind near 108–109 s, and reverse shock from the interstellar medium near 1010 s. Using one-dimensional hydrodynamics we show that the first creates a dense shell of Si and C atoms near 106 s in which the SiC surely condenses. The second reverse shock causes precondensed grains to move rapidly forward through decelerated gas of different isotopic composi-tion, during which implantation, sputtering, and further condensation occur simultaneously. The third reverse shock causes only further ion implantation and sputtering, which may affect trace element isotopic compositions. Using a 25 M supernova model we propose solutions to the following unsolved questions: Where does SiC condense? Why does SiC condense in preference to graphite? Why is condensed SiC 28Si-rich? Why is O-richness no obstacle to SiC condensation? How many atoms of each isotope are impacted by a grain that condenses at time t0 at radial coordinate r0? These many considerations are put forward as a road map for interpreting SiC X grains found in meteorites and their meaning for supernova physics.