I use an 80 grit disc on the DA, then go over it a second time with 180. Use a regular 6" DA on the bigger areas and a 3" on the smaller places, a sanding block where the DA won't reach. Angle grinders spin too fast, wear out the sanding discs almost instantly, and leave swirl marks on the metal. With the WD40 on it I'd suggest scrubbing it thoroughly with lacquer thinner, then a good wax and grease remover. DP 40 is a good primer, you should shoot paint over it within 24 hours, if not wet sand the primer with 400 before you shoot it with paint. I use a base coat/clear coat on a frame, just a bit more protection from chips.

Have you had the car all mocked up yet to be sure you don't have to do any changing to the frame? I like to completely assemble the chassis with all the fuel and brake lines installed and clamped along with any holes drilled necessary to secure electrical wires. Also a good idea to have all the exhaust cut, fit, and installed once to ensure you have all the necessary mounting points on the frame.

JMO, but I think a black frame is boring. Also with too much black paint on the frame and other components things start to lose definition. I prefer a contrasting color on the frame, or the same color as the body.