Per title, got a cast iron valley gutter in my industrial conversion, getting in for 100 years old now. May have a bit of a leak but generally ok condition. Not sure what the gasket material is between sections but the main gutter has surface rust and pitting. Two outflows, one at either end, in the floor of the gutter about 75mm away from the parapet wall and then a curved pipe directs water through the parapet. I'm planning on coating these internally too with the same stuff, probably by bunging the outlets, filling them up with the goo and then unbunging and catching the excess as it exits, so I'm thinking whatever I use will need to be pourable/flow but be viscous

I used Cromapol on the aluminium box gutter of my conservatory to seal over the join (applied to about 6" either side of it). It has bonded very well with no peeling after 6 or 7 years. I check it every year. I still have over half a tin full which I could use to re-coat if necessary, as it seems to last for ever in the tin.

https://www.amazon.co.uk/Cromapol-Acrylic-Waterproofing-Sealant-Reinforced/dp/B01MXJBQZP/ref=sr_1_5?keywords=Roof+Sealant&qid=1686434413&sr=8-5