Fuel tank sorted now I think, boshed a little bit of JB Weld 4 minute on the joint and it seems to have done the job
When I had my 50 I never had to paint the exhaust and I rode that in the snow and some of the saltiest roads ever. 125 however is a different storey, paint all the time or rust to pieces. It's already had a new exhaust when I got it and it was only 2 years old. I assume for the same reason.
I've not tried a high temp under coat, no, so I might give this a go when funds enter my wallet
Fuel tank is metal btw, wish it could be expanded a little