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 Painting Aluminum (Especially Outdrives)

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GeeBee Posted - Mar 16 2007 : 09:07:55
Painting Aluminum. It should be an oxymoron. But it happens. Especially aluminum out drives. I recently pulled my Mercruiser Bravo III. After 4 years of my paint job on it, not a single square inch of paint had eroded or peeled from the unit. Here is how I did it and some rules to remember when painting aluminum.

Number one. Aluminum will not take paint. Not bare aluminum, not etched aluminum? Surprised at “etched aluminum”? You see a lot of primers that say, “self etching”. Well etching is the process of using acids or bases to remove aluminum oxide and other contaminants. Where do the contaminants and oxides go in a “self etching” paint? That is correct, they are contained in the paint job. How long and how durable is a paint job with “self etching” paint after all those contaminants are contained therein?

Number two. Alodine will adhere tenaciously to aluminum and paint tenaciously to alodine. So alodine is our “negotiator” to paint aluminum. It provides the intermediary between paint and aluminum.

Surface prep. A lot of options here. If the paint job is original Mercruiser it comes off very easy because Mercruiser has a poor process. You can sand it off for the most part. If you want, you can get some “aircraft stripper” available at most automotive paint stores or Aircraft Spruce. The main difference here is the amount of ammonia in the stripper. What is critical in this process is to place several plastic bags over the input shaft and bearings and the prop shaft and carrier bearings and hold them in place with tape. If you have a nearby facility that does plastic bead blast paint removal, like an aircraft paint shop, they can clean off your drive as well. Just make sure to cover the pitot tube entrance on your drive. DO NOT SODA BLAST. You will make the etching process a very difficult one. Get the drive as best you can down to bare metal. DO NOT USE STEEL BRUSHES, STEEL WOOL, STAINLESS STELL BRUSHES, etc anywhere, any time. Always use aluminum oxide sandpaper and if you want a “brush” use Scotchbrite pad.

Etching. I use AlumnaPrep 33 available at Aircraft Spruce. You dilute it 1 part to 3 parts water. A quart bottle is more than enough for one drive. Follow the instructions on the bottle. USE RUBBER GLOVES. Brush the solution on generously. If you see a spot repel the solution it may have grease on it. Degrease it with acetone or MEK and put on more Alumnaprep. Leave the Alumanprep on at least 3 minutes then flush with fresh water. Use a hose. Flush like crazy. Your drive should have a dull but bright look to it after etching. Allow drive to air dry.

Alodine. Again available at Aircraft Spruce. Do not dilute. A quart will do a drive. Brush it on your dried, etched drive. Again, if you areas that are aluminum that repels the alodine, you go to de-grease. Clean and re-etch. Then alodine. Do not flush the alodine, allow it to air dry on. Your drive should have a nice “golden” tint to the bare metal.

Paint. Here is where you have some choices. I personally prefer two part epoxy polyurethanes. They are extremely durable, flexible (hard paints crack), and give an easy and very shiny finish that remains that way. The only negative is, any corrosion occurring beneath the paint surface will not be visible. The paint may bubble, but the coating will remain intact. Make sure you have sanded and etched away all previous corrosion before painting. I use Southern Polyurethanes. The other negative to polyurethane is you want a very good respirator and face shield when you paint. You can get one a Home Depot. Do not get the disposable kind. Get a 3M 4000 or 5000. Do not allow this stuff to get in your lungs. It does not come out, ever. Make sure your mask seals tightly around your face. If you have a beard, shave it.

Priming. I use Southern Polyurethanes Epoxy Primer. For a Mercruiser use black. Follow the instructions for mixing the hardener with the paint, (It is one to one). I use a small eight-ounce touch up gun. It just about covers a drive. First spray a flash coat, and then come back with a wet coat. After this when the wet coat is highly tacky and not subject to running, come back again with a full second coat. If you feel rich and want to burn some paint put on a third.

Zinc-Chromate Primer. A lot of people swear by this stuff. It is very good coating on etched aluminum, but not under water! It contains zinc! Think about that for a moment? There is one place to use zinc-chromate. On your bearing carrier after repainting the drive.

Color Coat. I use Southern Polyurethanes Universal Black, 4000 series. Again just like the primer, activate it one to one. Spray a flash coat then a wet coat. I do two applications. You’ll like this step because you’ll be amazed at how bright and shiny your drive is.

Clear Coat. You can (optional) clear coat your drive with Southern Polyurethanes Clear 4000 series. Do it just like the color coat. You can buff it if you want. Alternatively you can put on clear anti-fouling. You have to be prepared that clear anti fouling will “degloss” your finish and after a few weeks in the water, it looks like your drive is “cloudy”. Your underlying polyurethane finish is intact and is fine.

Refinishing. As I said, I recently removed my drive and the paint was intact, but I wanted to “clean her up”. I removed the clear anti fouling with #60 sandpaper. I roughed up the polyurethane finish coat with #60. In the process, I broke through to bare metal in a couple of spots. No problem. Etch those spots with AlumnaPrep, then alodine them. I sprayed two coats of Epoxy Primer, two Coats of 4000 black, then two Coats of Clear. Here are the results. Note the sheen near the bottom of the drive.

Sources of materials

43   L A T E S T    R E P L I E S    (Newest First)
jpowell Posted - Mar 16 2015 : 18:48:50
Still available and used.
Billylll Posted - Mar 16 2015 : 15:07:02
I guess Zinc Chromate is no longer available or used? I used it decades ago when painting anything Aluminum as a primer base. It looked like translucent Yellow when sprayed on Aluminum.
The color coats were done using Imron. Both paints were pretty dangerous to work with. Are either still used?
GeeBee Posted - Feb 28 2015 : 10:35:26
Update, after 9 years, my drive needed a complete refinish. That is 9 years in the water, not on a trailer folks. I began took me two 12 hour days to get it all off. The polyurethane finish coat was the one looking bad, and it came off fairly easy. The primer coat bonded to the alodine. WOW! Tenacious does not describe it. 2 gallons of aircraft stripper and 40 pounds of walnut hulls later, I got it done. In reality, if I was not so dissatisfied with the cosmetic appearance it could have gone years more on the primer coat alone.

Got it stripped, used Alumnaprep 33 then alodined it. I put on the same primer however this time I went with POR 15 Hard Nose finish coat. It is supposed to be even tougher than regular polyurethane finishes. I'll let you know how it goes, but I think this POR 15 Hard Nose looks promising. It does not finish as glossy as regular polyurethane but the point is longevity above all else.

woodshoes Posted - Dec 30 2014 : 13:02:30
Thanks Gee Bee, it sounds as if you have a background in metallurgy or aviation airframes. My VP's will be given your treatment this winter
alk Posted - Nov 21 2011 : 21:08:59
sorry Mike, just saw your question, not sure if you are still checking, but just in case - my drive turned 'yellowish' if I recall correctly, but it's been four years. The drive still looked very good three years later, when the clutch started slipping, and I bought a new drive.

in case anyone is reading this thread, I have aluminum window frames on my pacemaker - they look pretty rough - was thinking of trying this process again. Anyone have any good or bad experiences trying to paint window frames? less issues with being in the water than the drive had obviously, but more of an issue with sun.
mceranowski Posted - May 16 2011 : 15:05:39

I used the dupont alumiprep/alodine as well, did yours turn the "golden color"? Mine did not...I etched and flushed twice prior to using alodine. Thanks

btoran Posted - Aug 04 2010 : 05:54:11
Quick question.... Can the process above be used to paint trim tabs, rudders, shafts, and props? Our boat is on a mooring for the first time and a diver who spent lots of time scraping the running gear recommended I paint all running gear with some type of anti-fouling next year. He did say to not paint anodes. Any advice would be appreciated.
mkivbren Posted - Jun 07 2010 : 18:04:21
OP - excellent write-up!! I have some spot repairs I want to do using this procedure. How do you feel about prepping the alum and etching for spot repairs. I will probably have them replaced under warranty but I'm ready to go in the water and their not THAT bad. If merc wants to recoat - this will be the specification to follow. TIA!
rray32539 Posted - Apr 29 2010 : 22:09:01
Taking delivery on my boat in May, I may not need this article for awhile, but it was an excellent read and I will file it away for future reference. As they say a boat is a hole in the water in which you throw money, and I am sure I will use your advice when winter comes around. Great article, Gee Bee!
GeeBee Posted - Oct 23 2009 : 09:46:35
My pleasure, thanks for the kind words.
jpowell Posted - Oct 21 2009 : 12:24:19

Although we really get into it politically, i wanted to take the time to thank you on preparing this section on painting aluminum. I sed it 2 years ago and it was great.

jpowell Posted - Oct 21 2009 : 12:22:57
google it up. Lots of other mail order places also.
Tanqueray Posted - Oct 21 2009 : 07:38:44
I went to the Auto Parts Paint Supply & Body Supply shop yesterday. I asked for Alodine and they looked at me like I had 3 heads. Nobody knew what it was any ideas for a source?
artl Posted - May 14 2008 : 21:19:30
Is there something that will work on a small touchup jusr for protection 1"x1"? Sort of like the stuff they sell for cars.
GeeBee Posted - May 08 2008 : 17:50:20
Yes, salty you can. Paint durability will be a function of the paint coating itself, not of the adhesion factor however.
Salty Tricks Posted - May 02 2008 : 13:06:07
I know this thread has been around awhile but ....

I need to paint a hydraulic jack plate that my outboard gets mounted on. this looks like a good prep for the aluminun plate. can one use a different paint other 2 part epoxies? I have already received the Alodine and Alumiprep from Spuce

GeeBee Posted - Mar 19 2008 : 17:11:50
Rick, you have to etch aluminum period. Your paint job might look good, but it will not last.
KnottyBuoyz Posted - Mar 12 2008 : 14:34:00
Ok absorbed all the good advice here as I'm about to repaint my VP Duoprop O/D. I've ordered the factory VP Paint and primer (Duracoat). Thoroughly read the instructions and there's nothing there other than the obvious prep of the drive for primer and paint. I intend to give it a light sandblasting with walnut shells and if they can't cut the crud off I'll go with glass beads. I know there are some areas where I'll go through the old paint to the aluminum but after all I've read here (and what little I could find elsewhere) I'm wondering if I'll have to treat the bare metal? The other side of me is saying, damn the torpedos, blast the crap out of it and hit it with the spray bombs, dunk it in the water and see what happens!! *lol* Will post pics.
blr Posted - Nov 15 2007 : 14:56:50
Nice Procedure GeeBee, the only problem with it is that using any cartridge respirator for any two part paint (where a hardener is involved) is quite simply not adequate protection, the only proper protection for this type of paint is a fresh air system.
Capt_Rich Posted - Aug 16 2007 : 21:25:55
Alodine 1201 should be rinsed with clean water (after the golden color appears). Do not allow the alodine to dry on the aluminum. Alodine is classified as a carcinogen and must be handled carefully. Wear skin and eye protection. Do a search on Alodine 1201 MSDS for more detailed safety info.

GeeBee Posted - Jul 19 2007 : 18:54:26
I have no experience with DuPont so I can't say. I can only attest to those products I have been using and had good results with.
peteyo Posted - Jul 16 2007 : 16:52:30
GeeBee, great writeup! I've used DuPont 225S and 226S with similar results, albeit on a hull not on an outdrive or gimbal housing and above waterline. Of the 2 different brands, would you recommend one more than the other, or is it a matter of personal preference? I'm redoing a mercruiser outdrive and luckily don't have aquamak's dilemma. Was going to use the DuPont simply because I have it but may give the alumaprep a try per your reply. Alas, I'm another victim of the "grass is greener" syndrome I suppose!
GeeBee Posted - Jun 05 2007 : 09:54:24
Until people stop asking how to paint their outdrive.
FinnBill Posted - Jun 05 2007 : 08:23:46
I wonder how long this thread is going to be at the top of this forum?
GeeBee Posted - May 30 2007 : 08:40:57
Yes, there are. I don't think splitting is required. Plastic bead blasting a properly masked drive seems to be the ultimate prep to me. If you have access to such equipment, go for it.
zigzag Posted - May 24 2007 : 13:52:12
It seams that it would be easier to split the drive to strip and paint? Are there any issues with doing that such as stripper eating the seals, etc.?

Also, you mentioned that it is ok to bead blast the drive. Do you cover the shafts and any orifices other than the speedo pickup pitot, especially if it is split?
GeeBee Posted - May 17 2007 : 07:00:51
As to the Volvo casting, can't say, don't know but the Mercruiser casting works just fine with AlumnaPrep and in fact that is what they use in their "recommended process". As to rinsing Alodine, you can rinse or not rinse, I've found that as long as the metal is properly preped with an etcher, there are no salts so I don't rinse.

As to disposal, you're on your own. Each situation is different. Alumna Prep neutralizes about as quick as vinegar and about as easily. Alodine, you don't use that much.
aquamak Posted - May 15 2007 : 13:54:55
I have a few questions and comments especially for GeeBee. First, I am repainting a Volvo DP-SM outdrive. I took your advice and ordered Alumaprep 33 and Alodine from Aircraft Spruce and was prepared to follow your method however, the instructions on the bottle of Alumaprep 33 said not to use on silicon-alloy conating aluminum castings. I have the Volvo shop manual for the drive and it specificlally state not to use aluminum cleaners that contain flouride as it will cause "smut" on the casting and not allow for good paint adhesion and corrosion resistance. Alumiprep 33 has flouride in it and therefore should not be used on that type of aluminum casting. Obviously Volvo's outdrives (at least the SX and DPS models) are silicon alloy containg aluminum castings. I do not know about older VP's, OMC's or Mercruisers but it is worth investigating. I called Henkel Technologies who manufacture Alumiprep, Alodine. etc. and talked to their tech people. They confirmed that Alumaprep 33 does contain flouride and that it shouldn't be used on copper or silicon bearing aluminum aloys. They told me that their cleaner "Metal Prep 79" (also sold by Aircraft Spuce) does not contain flouride and cand be used on aluminum castings. I am sending back the Alumaprep and ordering the Metal Prep 79.
The questions I have are: how do you guys safely and environmentally use the cleaner and Alodine? How do you dispose of the was water and residual chemicals? Where do you actually do the cleaning and Alodine? Do you build some sort of lined enclosure, etc.?
Second question is that the Alodine instructions say to rinse (see next):
8. After Treatment:
Water Rinsing:
A thorough rinse with clean water is necessary to remove residual ALODINE 1201
coating chemical salts from the metal surface. Blistering and corrosion
problems under paint are often the results of poor rinsing. Chemical salts
trapped under a paint film will eventually result in blistering or corrosion

Are you sure you don't rinse?
Please let me know your comments/
GeeBee Posted - May 02 2007 : 20:07:42
Because most bottom paint contains copper, yes it causes huge problems. You set up a galvanic cell when you paint aluminum with bottom paint containing copper. Mercruiser says to leave a 1 inch gap between bottom paint and the gimbal unit. If you are going to paint bottom paint on an aluminum hull, it needs an insulating layer or non copper paint usually one that contains a zinc derivative.
Captstever Posted - May 02 2007 : 18:19:12
I haven't used any bottom paint on aluminum outdrives or hulls. I've been a fiberglass and inboard guy. I have heard that using the usual bottom paint, say Micron 66 or others used on Fiberglass, and painting outdrives with some aluminum exposure or any exposed aluminum causes severe problems because of the metal content of the bottom paint. This would include aluminum ladders that people use on their docks that spend most of their time in the water. Any thoughts?
GeeBee Posted - May 01 2007 : 17:38:02
Powder coating is very very durable. I powder coated my anchor about 8 years ago and despite a lot of abuse it looks great. I think powder coating should be the method Mercruiser uses to start with. That said, you have a problem with the aluminum casting in powder coating in that you have to bake on the coating and I'm not sure you can get the unit hot enough without effecting the aluminum alloy. Merc is very closed mouth about the properties of their alloy so without more information, I can't say that powder coating is appropriate.

As to what to do with your steel brushing that is a tough one. I would pressure wash the unit very vigourously especially in the areas where you brushed it. Then Alumna prep it.

Captstever, I assume you mean bottom paint on an aluminum hull? Or did you use it on your outdrive?
fasteddy Posted - Apr 30 2007 : 16:33:46
Great information GeeBee, thanks a bunch. What is your opinion on powder coating? A manufacturing friend offered to do this for me for free? And the really dumb questions. Now that I used a wire brush on the tough spots what do I do to correct the screw up?
Captstever Posted - Apr 28 2007 : 18:42:18
Great Article. Could you address what happens when people put the wrong bottom paint on aluminum in saltwater?
STEELA Posted - Mar 26 2007 : 16:39:24
I stripped down my aluminum tabs this weekend.. Used your model as well and it's working great.(thanks) My GD cat however jumped up on a "wet" tab, got in the boat, and tracked the primer all over the place!!

Any one need a cat?
GeeBee Posted - Mar 26 2007 : 16:35:17
Actually Ernesto I am sorry to say, that Volvo's paint job appear much beter.

Alk, glad it turned out good. I think you will enjoy the durability.
alk Posted - Mar 26 2007 : 09:42:06
I finished my drive over the weekend, GeeBee's process works very well, but a couple notes for anyone else who tries this.

If you don't want to mail order the chemicals, find an auto body supply store, that sells PPG products, look in the yellow pages. They have the Aluma Prep & Aldoline ( DX533 & DX503), the primer , and the polyurethene acrylic enamel ( DAR). They also have printed instructions for all of the products, and are generally helpful.

I used a plastic bag taped around the u-joint and upper shaft - which got wet when I rinsed off the aluma prep, and appartenly was still wet when I put on the first coat of primer. It dripped on the primer. For the next coat of primer, and the top coats, I taped a towel on top of a new plastic bag, and didn't have any further problems.

If you can find someone else that wants to paint their drive, do it together so you can split the cost of supplies. I only used about one third of the supplies I bought. I probably spent close to $200 at the autobody shop, and came home with enough prep, primer and paint for at least two or three drives.

If like me you don't have a lot of experience painting with a compressor, find something you can paint that you don't care what it looks like, to get the gun and compressor set properly, and to get a feel for how the primer and topcoat behave when sprayed. My rusted old wheel barrel looks pretty good in black, with a few paint drips.
Veg Posted - Mar 23 2007 : 15:57:00
OK GeeBee, what about us Volvo owners? How do we get the exact Volvo Grey? :-)

Seriously though, you said "If the paint job is original Mercruiser it comes off very easy because Mercruiser has a poor process." Would a Volvo be about the same?
GeeBee Posted - Mar 23 2007 : 15:44:54
PropMD in Minnesota. 135 bucks
4mula owner Posted - Mar 23 2007 : 15:40:04
Forgot to ask, what do you do to polish your stainless props?
KahunaDon Posted - Mar 23 2007 : 13:40:53
This is great GeeBee. I print some of the threads that are most important to me and archive them in a three ring notebook for future reference. This is definitely a keeper!
4mula owner Posted - Mar 23 2007 : 12:18:47
Thanks for the info. What would you recommend for small touchup areas (ie. the size of a pencil eraser)?
drpete3 Posted - Mar 20 2007 : 15:28:49
I thought a coat of poly is good on anything, Lungs and all
psugar Posted - Mar 20 2007 : 14:57:09
Great writeup. You are also so right about polyurethane never coming out of your lungs great safety point.

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