I tried both when they took the old EP-2 off the market. The KP-21 is formulated to strict VOC compliency, and last I knew of, they put this stuff out to meet the VOC requirements for California. I didn't like it, as it was hard as a rock to sand. I liked the KP2CF better from a sanding standpoint, but had problems with adhesion both times I used it. I dumped the HOK epoxies for SPI, which I now believe to be the very best epoxy on the market. I've tried many, not all, but this stuff from SPI is just awesome!!! It sprays great, sticks like glue and sands very nice!