[Uncle Dave]

Greetings,I have removed all the foam from a Sylvan 21'. The alum hull is badly corroded on the inside. White slimey mess. Still wet from rainwater?. Seveal holes (one inch) in the hull. They are repairable.The bigg question is how to remove the corrision and treat the alum before repouring the foam.Maybe acid and then zinc chromate primer. Nice boat, just abused by a previous owner.Thanks,

[Lark40]

You won't hear this from anyone else, but I prefer to sandblast corrosion from aluminum, using common play sand. Have had 100 percent success and long-term satifaction. Stay away from using acids.The next question is : what caused your corrosion?Foam didn't do it - was it electric current or contact with a more noble metal? If you can't resolve the cause, you won't prevent future problems.

[Uncle Dave]

Lark,Dont know the exact cause. I checked the wiring as I dissamabled the cuddy and removed the deck. Battery was properly connected and in a plastic box. Water had entered between the foam and hull. The deck was not sealed where it meets the sides. Therefore rain and sea water drained to that area. The deck was not rotten. When purchased it was under a very leaky tarp and had tree leaves on the deck. Worse and most corrosion is not close to a rivit. I have read of using wallnut hulls to "sandblast" with. Not sure of any organic leftovers. Clean play sand sound like a good idea.What do you suggest to prime with after sandblasting? The out side of the hull looks fine except for several holes that have eatten thru from the inside. I plan to only touchup paint the outside of the hull.

[Trent]

After you mechanically remove the corrosion you WILL need some chemical protection. When you blast you remove the protection. Alumiprep and then alodine the surface is the correct way. Then prime with zinc chromate primer. I really dont understand why Lark said not to use acids?? They are used everday in the marine and aircraft industry. If you dont use the alumiprep/alodine at the very least prime with zinc chromate! Corrosion Removal

[Lark40]

Trent,I have seen what damage the wrong kind of acid can do to an aluminum kitchen sink. So, I just tend to avoid it. Of course, if you are familiar with the right stuff, no problem.The sand blasting leaves a good "bite" on the surface, so I have never have to use primer. My boats are trailer boats, either used in fresh water, or washed after salt water usage. The lack of primer doesn't seem to affect my boats. Mr. Fitz should follow your advice for more typical use.

[Uncle Dave]

Greetings AllThanks for the advice on the corrosion. I have some concern about using the acids. The hull is a vee hull with ribs every 12 inches or so. They were constructed to be water tight perhaps, but with pop rivit holes the acid can get under the ribs and can not be flushed very effectively. Would not this cause rapid failure of the hull under the ribs.Would a plan to sand or bead blast and immediately prime with zinc chromate primer work as well. Of course a pretty finish is not required. Finaly should an additional sealant be applied over the chromate primer such as bed liner, gluvit etc. Or just re-foam after the primer?Mr. Fitz.

[DJ]

Mr. Fitz,I would definetely use Gluv It and put it over the Zinc Chromate.