True Grit Aluminum Build

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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Roberta
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Location: East Troy, Wisconsin

Re: True Grit Aluminum Build

Post by Roberta »

When I painted my RV-7A I cleaned a lot with MEK and scuffed with scotch bright. An epoxy primer was used with a two part polyurethane topcoat. Good adhesion and still looked great after 5 years when I sold it. Still good as far as I know. after 13 years. The aluminum was ALCAD. Don't remember what grade.

Roberta
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Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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kens
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Re: True Grit Aluminum Build

Post by kens »

As Kevin mentioned, a mechanical abrasion bond is the best. The OP mentioned sanding 180 grit, Kevin say bead blasting, I agree with both, or either.
And of course with acid etching, yes.
The least desirable is chemical such as MEK/ScotchBrite. MEK is a solvent such is lacquer thinner, naptha, toluene, or NYS (name yur solvent) and it merely dissolves oils/impurities but doesn't get rid of them, when the solvent evaporates the junk is left behind. Not to mention once the junk is dissolved on yur rag you just smearing it around. Many people use MEK in the small EAA aircraft industry and claim success. Perhaps their item is stored in a closed door hanger and climate controlled, and only flown on sunny days. That ain't gonna work for a boat in the water.
Please allow me to introduce another option. ScotchBrite/Ajax. Ajax as in the grocery store powder + old school RED scotchbrite. Green scotchbrite don't get it, it has to be the old school red. Use enough potable water to make it suds, scour it in, and you can cut materiel on par with 180 grit.
No chemical solvents here. Rinse it off with potable water. The soap capacity of Ajax will lift oils/dirts, it will scour, and you rinse it off, potable water pure. MEK and yur other solvents are considered haz-mat. Ajax/water is not.
Dry it immediately, and get a epoxy primer on it. you're done.
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

Kevin Morin
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Re: True Grit Aluminum Build

Post by Kevin Morin »

Kens,
sanding roller, rotary or even hand applied compared to bead blasting or sand blasting aren't equal in my experience. Sand blasting or media blasting performs an impact anchor pattern that will allow film coatings to 'grip' more than the raised profiles of all rotary sanding media, and this is confirmed & supported online by the many anchor pattern articles I've found.

If aluminum is not etched of its aluminum oxide- when the sanding/blasting or other treatments happen- the naturally forming aluminum oxide film will for in a couple of seconds of exposure to the oxygen in the atmosphere. When this film has formed; then the underlying metal is only protected by the adhesion of that chemical film- and it is well known to give up when wetted by acids or bases- wetted by liquids of either extreme ph.

However, if an acid or compound of acids are applied they can remove the aluminum oxide film and then be rinsed by fresh water- keeping the surface wet will exclude O2 from the ambient air from supporting the formation of aluminum oxide. By flooding the wetted (rinsed) surface with Alodyne, a chromium solution, a layer of chromium oxide forms and bonds to the underlying aluminum and forms a protective layer superior to aluminum oxide in chemical and corrosion resistance.

This results in a superior paint base for aluminum. By coating an etched aluminum assembly- boat or tank- with Alodyne then applying the primer to that coating- the adhesion is much better than what will happen from aluminum oxide- and much longer lasting and durable.

Alclad is not marine aluminum. So the properties of painting aren't the same (not identical), perhaps similar in some aspects but my experience painting airplanes and boats isn't the same. While similar- not the same. 5000 and 6k series alloys surely aren't the same as Alclad! I did lead a crew that stripped a Cessna 206 for painting, and we did use acid etch, allodyne and then turned the ship over to an aircraft painter. The prep worked, the paint 'stuck' but it sure wasn't the same as the boat's we've painted.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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Roberta
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Re: True Grit Aluminum Build

Post by Roberta »

Thanks For clearing that up, Kevin. I was unsure of what alloy aluminum was used in boat building. I assumed it was also alclad. While my method resulted in a superior long lasting aircraft finish, it may not be the best for an AL boat.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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kens
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Re: True Grit Aluminum Build

Post by kens »

Alclad in aviation world is pure dead soft aluminum plating.
The aircraft skins or parts are 2024-T3 or 7075-T6, 7075 especially, corrodes.
Pure aluminum is least corrosive thus it used as the cladding.
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

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polynimbus
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Re: True Grit Aluminum Build

Post by polynimbus »

After some amount of paralysis-by-analysis, I finally cut the giant hole in the boat. I heeded Kevin's advice and went with 1/2" wall tube, and left plenty of ID to accomodate anything from 1-1/8" to 1-1/2" shaft (most scenarios are calling for a 1.25" shaft but possibly 1.375"). Cutting a 3" hole through the keel was intimidating, but it worked out pretty well.

I made a steel fixture with delrin bushings to guide the drill rod, and just attached that to a hole saw. Clamping to the skeg this way fixed the athwardships position, so the only thing to adjust was the angle (that was just accomplished with blocking and an old fashioned ratchet strap). The whole set up was surprisingly effective, I re checked angle and plumbed the projected shaft about 30 times just out of paranoia, but never moved.

The only downside to using 1/2" wall tubing is that it required preheat in order to get a good root weld. That took some practice, as I couldn't get to both ends evenly since one was outside the boat and the other inside. Heating from one side/end measurably moved the tube (even with shims) since there was so much thermal mass. Eventually I got an order of operations that would allow even heating and allowed me to weld it in the exact right place.

Definitely glad to have that step behind me, now just need to do the (much simpler) rudder tube. And we will be about ready for bottom paint.

-Joe
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polynimbus
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Re: True Grit Aluminum Build

Post by polynimbus »

One more note, this was a great opportunity to "destructively" test the keel-skin weld joint. The hole saw approach keeps the slug intact, so I could bend the plate until failure and check penetration on the cross section. This picture shows one of the test slugs and you can see how easy it is to see the weld penetration, even before using penetrant (don't mind the drill marks, those were required to get the sections cut off the boat).

-Joe
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kens
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Re: True Grit Aluminum Build

Post by kens »

Very nice work
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

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