yes POR 15 is an excellent paint for the outside, but the rust on this part of the frame is caused from the inside out.

I would recommend you also inject some Dinitrol cavity wax, and also ensure there are drain holes especially at the bottom of the curve. If there are none, which if a new tube is welded in there probably wont be, then drill a couple, paint the drilled openings with por 15 but ensure they remain open as in not blocked, then inject cavity wax. This should ensure longevity.

Mine is passing MOTs ok at the mo but I know there was some corrosion in the lower bend at the centre of the subframe lower crossmember, but I cought it a few years ago and injected wax plus did the outside with dinitrol black and so far so good. One day though, it will need doing and I will probably go down the welding route at the MGOC garages or somewhere with experience of them.

If anyone does source and fit replacement rear subframe, please take the time to paint it and wax inject it all over as above to save yourself future hassle, and ensure there are clear drainholes in sensible places wherever water may collect and sit.

I also wax injected the front subframe whilst I was at it.