repairing cast iron gutters and drainpipes

I have a 1953 house with the problem you describe. However, I also have a scaffold tower with wheels that reaches all the gutters. If you feel at the very back of the underside of them you are likely to encounter a crumbly layer, ie corrosion. I scuffed this off by hand and wire brushed where accessible. I googled 'Hammerite' and bought the cheapest black 5 litre version plus lots of 3" cheap brushes, as it's not worth cleaning them, the price they are. I painted the underside and topside using plenty at the back problem area. I removed the down pipes and found most were showing signs of a 'split' down the back. I filled these with 'gun and nails' smoothed off by hand and then painted with Hammerite. All the wooden spacer bobbins were rotten so I replaced them with plastic. The nails I replaced with galvanised 10mm bolts using plastic rawlplugs. Admittedly a lot of work but hopefully not repeated in the next 10 years +.