Greetings,
I have used marine grade epoxy with great success, but the tank needs to be clean, I use a rotary wire brush.
I have used it on 71/4 gauge ride on tenders, with no problems.
Marine epoxy is a bit more runny than araldite, but needs 20 to 25 degrees to set properly.
It can be stiffened with various additives, if you need a paste, and from your local yacht chandlers you should be able to get the "West system", which is the most well known supplier.
It will seal over the solder, and during its working life remains a little flexible.
Regards
Brian