Aluminum dinghy hull

[downwinder]

My dinghy stayed in the water for a while. Any advice on that os the best way to clean the hull and not damage the rubber vee. Thanks

[Capt Clark]

Downwinder,

It looks like a job for Ospho or similar product. You will want to use gloves.

Capt Clark

[tkeithlu]

From the picture, I'm guessing that the hull was made of Alclad, an aluminum alloy coated with aluminum. Pure aluminum self-protects, but is too soft for structural use, so aluminum alloy, which does not self-protect (it corrodes) is coated with pure aluminum. Those round splotches look like places the alumium coating has failed.

I suggest that you clean it up with Scotchbrite, which is a sponge with fine carborundum imbedded. Get the real stuff, not the version 3M put out for kitchen use.

Then you prime and paint it, or you hope that manateeman, a forum member who owns an aluminum boat, comes on and tells you about the best protection for bare aluminum.

Here's Scotchbrite:

https://www.amazon.com/Scuff-Pads-Sc...s%2C169&sr=8-6

[Jim Cate]

Not a great photo, but the spots look to me like barnacle "footprints"... the residue left after you mechanically remove them. Is that the case?

And Tim, I've never heard of using alclad for boat building... it is mostly an aircraft construction format in my understanding. Most alloy boats are built from a 5000 series alloy which has good saltwater anticorrosion properties.

The footprints can be removed mechanically as Tim suggests or with mild acid products.

Jim

[downwinder]

Greetings. Those spots i can remove with my fingernail. Nothing mayor. Just wondering what would be soft on the rubber vee the aluminum bottom has.

[downwinder]

[noelex 77]

Aluminium RIBs are normally made from one of the 6 series aluminiums such as 6061. While not as corrosion resistant as the 5 series aluminiums, it is usually left bare on the outside. Paint or other coating is used on the inside for aesthetics and non skid properties, but this is not otherwise needed or beneficial.

As the bottom is raw aluminium there is no danger of damaging or cutting through any coating such as the gelcoat on fibreglass ribs. So using even aggressive mechanical methods is fine and this would be safer and more effective than a chemical approach that risks damaging the rubber vee.

Keep in mind small tenders are constructed from very thin aluminium to keep the weight down. Generally 3mm or less thick. It can still be attacked with techniques such as flap wheel on an angle grinder if needed. The material is more abrasion resistant than fibreglass or plastic. However, the growth shown in the photos should be easily removed with Scotch-Brite pads and hand "sanding". Beware of using anything with metal such as a brass wire wheel, this will leave behind tiny pieces of dissimilar metal imbedded in the aluminium that can cause corrosion.

[Manateeman]

Be very, very careful with the area where the fabric meets the aluminum. No chemicals or acids or you will have a difficult repair.
Wipe the entire surface with acetone.
Hand sand the barnacle rings off. NO 36 grit on a grinder...it’s aluminum not fiberglass. Keep removing the sanding residue as you go.
You don’t want to grind stuff into the aluminum.
Wipe again with acetone.
Time is not on your side as you do the final preparation because aluminum wants to self coat with oxide. Get a new stainless brush and mark it alu only.
You can use this and a credit card to get close to the fabric.
You want clean aluminum. You don’t need a tooth of deep scratches but you are not polishing it either. Do a small area above the waterline on the transom to pick the best grit. Brush the welds a bit more vigorously.
Once you get the bottom prepared do one more light wipe /sand/ wipe sequence while your assistant prepares the barrier epoxy. We like the total boat stuff but there are a lot of other choices.
Obviously don’t use copper antifouling. We are trying the new Sherman Williams but it’s too early to comment.
The above assumes your boat was not powder coated. You also might want to use a chromate or other prep coating. It’s interesting to talk to aircraft people as they really do a good job with aluminum paint. But more complicated prep but the basics are the same. Don’t grind in dirt, oil, grit ...stuff.
We’ve added a keel guard but I’m not sure it works in all beaches. Your dinghy is not a landing craft. The interior needs protection as well or your scratches will become pits in short time.
Happy trails to you.
The manatee crew.