And I'm happy with the outcome:
Above: passenger side body filler smoothed, rocker sanded for painting.
Rubberized rocker coating.
Painted - one nice thing about a black car: easy to get a colour match.
The only obvious sign of the repair is the tape line on the dogleg where the rubberized layer stops. The OEM coating ended lower down and was blended instead of cut off. But if you didn't know the car, you might not notice.
I'll spray some more rust inhibitor inside the rockers for good measure, re-undercoat in the wheel wells and that will do.