09/07/2006, Long Beach Shoreline Marina, CA
Figure 41 - High pressure spraying of Strontium-Chromate primer after acid etch preparation. Strontium-Chromate is the standard used by the aircraft industry for corrosion protection of aluminum parts. If you can get the stuff, we highly recommend using it. Within 24 hours of priming with Strontium-Chromate, it is possible to apply high-build epoxy primer without additional surface preparation. The high-build epoxy primer will cover all scratches and pits caused by corrosion and provide a glass-like finish for your top-coat of LP.
Indicative of an appropriate cleansing and etch is a bright aluminum surface, on which the liquid sheets completely, where after fresh water is used for a complete rinse. Any areas where beading is noted (as on a freshly waxed car) should be retreated. The surfaces are kept wet with water until the conversion coating is applied. Surfaces should not be allowed to dry during etching operation.
ALODINE 1201: This chromic acid conversion coating is a long time standard, endorsed by Sterling and other noteworthy aviation coatings manufacturers, is applied full strength by plastic sprayer, sponge or rag over a wet surface, following acid etch application and clean water wash. Alodine1200S powder is also acceptable, diluted to manufacturer?? s recommendations with water. The Alodine solution changes the bright aluminum to a transparent watchband gold color within a few minutes, following which another water rinse is undertaken. Do not allow the aluminum surfaces to turn a dark brown color as this excess coating will preclude appropriate paint adhesion.
Primer should be applied as soon as practicable after rinsed surface has dried. DETCO U-1201 yellow, strontium chromate epoxy primer is mixed 1:1 with U-1202 catalyst and should pre-react approximately one hour before thinning and application. U-1201 meets military aviation specification MIL-P-23377D and is an excellent anti-corrosion coating over chemically-treated aluminum or steel. An aircraft standard, U-1201 is used as a finish primer for aluminum skins, components, floats and interior surfaces, demonstrating excellent adhesion and anti-corrosive protection.
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09/07/2006, Long Beach Shorelione Marina, CA
Figure 40 - Removal of the bottom 1” of mast to eliminate corrosion and make room for the new mast foot fabricated from Duraform-GF.
If you are ever planning on repainting your aluminum spar, the most important aspect of the job is the preparation of the raw aluminum before priming and the type of primer used. Once you have the raw aluminum sanded in one direction with 180 grit, it is necessary to properly prep the aluminum before priming for paint. There are many steps involved and they should be followed exactly or face having to redo the job in a few short years.
Aluminum Preparation & Priming Method for Aircraft by Sterling & Detco
For best results, all aluminum parts should be lightly abraded prior to surface treatment, which consists of the application of an acid etch followed by conversion coating prior to the application of anti-corrosive primer. This pre-treatment process greatly improves adhesion and corrosion resistance. ALUMIPREP 33: Sterling and other noteworthy aviation coatings manufacturers strongly endorse the use of this product, manufactured by Henkel as a most effective material to use. Alumiprep 33 is an acid etching solution, designed to create a microscopic profile in the metal and at the same time, to cleanse the surface of contaminants, removing oxides of aluminum in the process. Usually diluted 2:1 with water it is best applied with a plastic scrub brush or abrasive pad over properly stripped and detailed aluminum.
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09/07/2006, Long Beach Shoreline Marina, CA
Figure 39 - Stripping the spar and preparing to apply Strontium-Chromate primer. Finish sanding using 180grit disks and a 6” Mikita high speed grinder makes short order of the job especially when you have a really big spar.
The Spar Refit actually began over a year ago when we un-stepped the mast in preparation for the deck project. When we un-stepped the mast oh-so-long-ago, we had it trucked from the shipyard to a construction yard several miles away on a large flatbed for $350. We stored the mast for far less than it would have cost to keep it at the shipyard. As most boat owners know, shipyards usually want $1/ft per day for spar storage which is ?theft” by any other name.
Once we finished the cockpit, the spar project began with the removal of all the hardware, tracks, winches, cleats, masthead etc. Like a broken record, what seems like it should be a simple task is actually very difficult and time consuming. The primary complication results from what is known as ?Galvanic Corrosion” between aluminum and steel. In the simplest of terms, galvanic corrosion, often misnamed "electrolysis," is one common form of corrosion in marine environments. It occurs when two (or more) dissimilar metals are brought into electrical contact under water or in a moist environment. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Either (or both) metal in the couple may or may not corrode by itself (themselves) in seawater. When contact with a dissimilar metal is made, however, the self corrosion rates will change: corrosion of the anode will accelerate; corrosion of the cathode will decelerate or even stop. In the case of our aluminum mast with hundreds of stainless steel fasteners, we had a situation in which all the fasteners had become galvanic-coupled or welded in place by the formation of aluminum-oxide, Al2O3. Aluminum-oxide is extremely strong as it is the basis of sapphire. Breaking this bond requires extreme measures!
The driving force for corrosion is a potential difference between the different materials. The bimetallic driving force was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor. The principle was later put into a practical application by Alessandro Volta who built the first electrical cell. This is the basis of all modern wet-cell batteries, and it was a tremendously important scientific discovery, because it was the first method found for the generation of a sustained electrical current. So, in other words, when you put aluminum and steel into contact with each other in a moist environment you?? re essentially producing a battery.
So, in order to break the bond between the stainless steel fasteners and the aluminum, it requires a propane torch and a pneumatic impact driver. The torch is used to heat the fastener and cause it to thermally expand, thus crushing the aluminum oxide. It typically requires about a minute of heating for each fastener and a few minutes to cool before using the impact driver to free the fastener. The process is slow, but it works really well.
After a full weekend of fastener extraction and removal of hardware, we then applied chemical paint remover and allowed it to sit over night. The following day we used paint stripping knives to remove most of the old LP and primer. To finish the paint stripping job, we broke-out the man-sized tool, i.e., the 6” high speed Mikita grinder. This has become our favorite tool! Using a 3M soft sanding pad and 60 grit disks, it only require a few hours to completely strip the mast, boom and spinnaker pole of all paint and corrosion. The 60 grit disks were then followed by 180 grit as a finish prior to priming with Strontium-Chromate.
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09/07/2006, Long Beach Shoreline Marina, CA
Figure 38 - Port and Starboard cockpit shoulders with winch platforms varnished before mounting of winches.
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09/07/2006, Long Beach Shoreline Marina, CA
Figure 37 - Photo of the port and starboard cockpit shoulders which required all the steam bending to precisely match the fiberglass profile from the top of the doghouse to the winch platforms. When shaped, sanded and varnished, it is nearly impossible to distinguish the laminated portions from the solid 1” teak boards. There are no visible ply lines!
Each of the four laminated planks was allowed to harden over night before removal from the tool.. When removed from the tool, there was 0% spring-back and the mould-line of the cockpit shoulder was matched perfectly. The laminated boards were then shaped, sanded and fit with winch platforms and fastened to the shoulders of the cockpit. After 5 coats of varnish, the new Lewmar 2-speed self-tailing winches were installed. Finally, we are putting hardware back on the boat!
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