Hmm, I think the weak link here is the protection afforded by the wax. P21S looks very nice, but it's got extremely limited durability, and offers little in the way of sealing the finish properly. If the paint can be corrected well using the PC then no need to switch to a rotary, but I would recommend swapping to a more durable final coating. Given the speed at which old solid red paint oxidises and turns milky, I'd suggest taking a look at sealant systems, and in particular the Werkstat Acrylic system. This is easy to apply and a lot more durable than the P21S wax. If you layer it up, you build up an acrylic skin on the paint that will really help to keep oxidation and milkiness at bay. More information on this link for you...

http://www.polishedbliss.co.uk/acata...-kit-kits.html

If this kit didn't do the trick I'd then be considering a respray if you love the car; if Werkstat Acrylic didn't effectively seal it then nothing will!