I did just what you are describing. I wire brushed the chassis with a high-speed angle grinder, sprayed on a good degreaser, power washed, and then painted with POR-15. I'm happy with the results I got. No doubt the results would be even better if you blast the frame, but, like everything else, it all depends on how much you want to spend. I chose to put my money into the important mechanical parts (engine and brakes), because I plan to drive my Bolt.