This may be old news to some, but, on the aluminum boat I have recently taken over we found damage to the hull from some improperly stored lead ballast stored aft of the rudders. The hull in that area is 15mm thick and in some spots it was pitted to only 7mm thick. We were looking at having to cut out part of the hull and replace it. Estimated cost $15,000 to $20,000. Then a friend told us about: http://belzona.com/products.aspx http://belzona.com/marine.aspx We made some calls and found out it was was ABS approved and would pass survey. To make a long story short we had it done, it only took three days and the cost of the prep and repair came to under $5000! Here are some pictures of the repaired areas.
After the metal was soda blasted they applied a structural material to fill the pits then over coated it with the more paint like product.