Painting Aluminum Drive Parts

I am trying to determine best method for painting cast aluminum drives below the waterline.

I have a cast aluminum surface drive that I have been trying to determine the best paint system.

I would like it to be a gloss finish such as on a new outboard. Ihave Awlgrip primers, Awlgrip jet black topcoat. I also have Randolph Paint's Milspec Epoxy Zinc chromate primer and epoxy topcoat. But I am not sure which one to use.

I have looked at several options to include soft anodizing / zinc rich chromate primer / epoxy topcoat or alodine conversion coating / zinc rich chromate primer / epoxy top coat.

Unfortunately I have been advised that you cannot alodine or soft anodize cast aluminum...so back to the drawing board.

I have sandblasting equipment and I am trying to determine the most resilient finish for cast aluminum below the water line.

The milspec solution from Randolph paints:
P23377 Strontium Chromate Epoxy Primer
PRF 85285 Mil Spec Black Top Coat

The ALWgrip solution: AWLgrip Ultrabuild epoxy primer, AWLfair, 545 Surfacing primer, AWLgrip jet black topcoat.

I was advised not to use a zinc rich primer below the waterline so I am a little lost

Someone also advised that even after sandblasting I need to acid etch.

I have alodine but I was told you cannot use it on cast aluminum. To complicate things, the aluminum drives have bronze bushings, so i am not sure if I can alodine the bushings with the drive (taking them all out would be a serious pain)


(Q1?) Any ideas for a paint schedule for raw cast aluminum below the waterline. The boat will be on a hydro lift and the other on a trailer so I would like something with a gloss finish.


Thank you

 

Have a look at this article by David Pascoe on the care and protection of aluminum drives: http://yachtsurvey.com/SternDrive.htm

Several of the big marine paint companies sell coating kits designed specifically for aluminum drive legs. These would include compatible prep solvents, primers and finish coats that are meant for this purpose. It's a bit different than ordinary bottom paint or topsides paint.

 

See the Interlux web site. Find a substitute for their "Viny Lux Primewash". It's expensive and probably just zinc chromate primer.

Easy Rider

 

If you sandblast, do not use silica based media. Walnut shell, bicarbonate of soda, or corncob maybe. The silica imbeds microscopic particles in the surface and can lead to paint failure. Silica sandblasting is a definite no no for eventual anodizing for the same reason. Soda is the best option because it is soluble in plain water and vestiges are easily removed.

 

Look at chlorinated rubber paints. It may sound a but weird because they are mainly used for pools, but there are other applications as well, like the lettering on lorry tarpaulins, painting inflatables and it is also used on offshore platforms.

It is not expensive, adheres well and stays flexible even after years of exposure to the elements. Available in several colors, but it is definitely not glossy!

 

Aluminium is a real pain to get good adhesion on, primarily because it is self anodizing when in the presence of oxygen (read air or water).

The best way I have found to paint aluminium is to clean it thoroughly only sand blasting if necessary, wet sand it with epoxy (use 80 grit sandpaper and unthickened epoxy instead of water), then once the epoxy has kicked paint.

Other than that the acid etchers will work, but ONLY use products designed for aluminium, and be fanatical about following the manufaturers dirrections for application.

 

Slightly aside from the paint issue, but on the corrosion problem: The shaft bearings in the moveable part will create galvanic connections from the propeller and bronze ball through the water to the Al. All the galvanic current will pass through any minor scratch in the surface protection on the al-parts.

To reduce this, I would use a bellows around the bronze ball, check if it is possible to mount the bearings in an isolating sleeve (or use isolated bearings) a.s.o. And finally, don't forget to install sacrificing anodes (Zn, not MG).

BTW, amazing that someone produces stuff with this kind of material combinations, and call it marine equipment!!!!!