Rusty roof sheets on overlap

Hi All.

Discovered this on our shed. Most off the overlapping sections are rusting out about 200 mm from the end of the sheets. Obviously it holding water but not sure why. I think it must be holding moisture at the overlap, but why only at the one spot i don't know.

Anyway, what the best way to fix this? short of replacing all the sheeting. The top section of the roof has been replaced as it was rusting out. The roof is 40 odd years old.

Have watched many YouTube vids on how to's, but still undecided.

Cheers

Attachment 511958

Rusting on gal sheet joins is usually caused by electrolysis eating away at the gal coating. Dissimilar metals will also do it. Is there a flashing under it causing the problem?

Re -roof. Or lift the upper sheets, clean off the rusty parts, treat them with converter, and paint them then re-install the top sheets, and do it all again in 2 years. Easier to re-roof and not have to get back up there for 30 years. Try and get full length sheets with no overlap.

Many converters will remove the remaining galv and can make things worse.
Reroofing is about it.

Correct me if I'm wrong but it looks like you have replaced the gutter at some stage and going by the internal gutter brackets I think the gutter is zincalum and the roof sheets gal. These 2 hate each other and the splash from the gutter hitting the gal will be enough to set up an electrolytic action, also the age of the roof sheets would indicate they are at the end of their life anyway.
Beardy has also mentioned another possibility

Judging by the soiled appearance of the end of the iron, might I suggest that the gutter has been full of leaves etc. and built up onto the roof.
Looks like you have cleaned it out and discovered the rust.
Just a thought, but replacing the iron would be the best solution.

John.