It shouldn't be necessary to take the whole frame back to bare metal. Treat the corroded areas by removing as much rust as possible then applying some Kurust, the gel type would be best. Then feather edge the repairs by working your way up in sand paper grit and enlarging the area slightly each time. You want to create a smooth transition from the good surface, right across the reapair to the other side. Work your way up to 400 grit dry, then key the whole thing with 600 wet and finally 800 wet. Spay on some 'isolator' such as Barcoat to stop any reactions between the previous paint and the new stuff. Then some grey primer, starting with the repaired areas to build them up a bit, let it dry very well, wet sand 800 grit untill it's all smooth, check for imperfections then move onto the colour coat. Hope that helps, any questions just ask