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Author Topic:   Corrosion and antifouling paint
Hank posted 05-10-2001 12:07 PM ET (US)   Profile for Hank   Send Email to Hank  
What is the best antifouling paint to use on submerged aluminum engine parts ? I keep my boat in the water for several months at a time. The bottom of my Tilt/Trim mechanism is very close to the waterline. When I had the bottom paint reapplied on my Montauk the shop also coated the lower ends of the engine bracket and tilt/trim cylinders with a black antifouling paint. I now find the paint puffing up on these surfaces due to a build up of what appears to be white aluminum oxide. The same paint is used on the lower edge of the skeg and cone.
The bottom paint on the hull works well. Only the special paint used on the metal parts is giving me this problem.

What type of paint does best for this application ?

lhg posted 05-10-2001 01:01 PM ET (US)     Profile for lhg    
I've always thought it to be "ULTRA HACK" workmanship to bottom paint the engine bracket components and drive sections. I've even seen propellers bottom painted!

The engine manufacturers say to never bottom paint the engine parts. They have galvanic corrosion inhibitors (either zinc, aluminum or magnesium) already provided. The boat has to be pulled out once in a while and these elements cleaned of algea, barnacles, etc as needed.

bigz posted 05-10-2001 01:32 PM ET (US)     Profile for bigz    
Larry has a good point and I follow that advice but I do know what the Florida waters can do to a motor -- that said --

Hank depends, will go into that in a moment --- first your going to have to strip all that original antifoul which by the way should have never been used off the parts.
(my boat yard just tried to convince me to do the same down in NC with the motors on our one 27 -- told them absolutely not!)

Now you can go one of two ways, you can clean up all the oxidation meaning sand it off so the metal is clean of any corrison, then use a zinc chromate primer about two coats then paint the bracket/t&t plus the lower unit the original color use Tempo spray paint cans for all and that's it (just keep it waxed and when and if it starts again do the same as above)keep the motor raised for best results while at "rest". The t&t's are always a headache the next idea maybe better for it.

The second way is to purchase either Interlux Micron 33 spray for outboards plus primer and follow directions or Tempo has a similar one in clear also a special primer is needed with it. These just go over the original paint on the bracket, t&t and motor. Both are illegal in CA and MI I think, and can only be used on outboards and outdrives.

Hope this helps, I like just to keep the original finish keeping after it yearly with touch ups which seems to work out ok ---

Hank posted 05-10-2001 07:10 PM ET (US)     Profile for Hank  Send Email to Hank     
Thanks for the advice. I was pretty much resigned to having to remove all the layers of antifouling paint from the surfaces in question and clean up any oxidation. I may try the zinc chromate primer and the Tempo spray paint to try to restore the original finish. I am more concerned about the corrosion than the fouling. Sounds like the best antifouling paint for engine parts is no antifouling paint.
Dan posted 05-10-2001 08:46 PM ET (US)     Profile for Dan  Send Email to Dan     
I used the Tempo clear stuff with primer. The tin content is about 2.9% -- I gave it quite a few coats. Masked off all areas I didn't want painted. However some people with more experience said I shouldn't use Tempo product, and should use Petit or something with a higher tin content. Same people said never use the bottom paint on engine parts.
tarbaby posted 05-10-2001 11:48 PM ET (US)     Profile for tarbaby  Send Email to tarbaby     
I think if you put copper and aluminum in salt water you essentially have a battery. By painting a lower unit you are actually causing more harm than good. I am sure that someone will be able to set us all straight on this. My 2 cents= Do not paint any part of an outboard!! Shay
bigz posted 05-11-2001 06:12 AM ET (US)     Profile for bigz    
If one feels anti-fouling paint is needed NEVER paint the zinc alloy anodes on your outboard, on your trim tabs, or on the hull (this warning includes Dyna-Plates -- the electronics grounding plate) you will reduce the effectiveness as a corrison "prevent-er" and ground element 70 to 80 percent rendering them almost useless.

Other than the above warning painting your outboard with either Tempo or the Interlux Micron will help prevent excessive marine growth.

I as stated above in my mind just regular maintenance with out the use of anti-fouling paint is a less costly and better way to approach the problem.

On zinc alloy anodes make sure as they wear, at say the 50% mark, you replace them, probably one of the least taken care of items on an outboard yet the best protection from galvanic corrosion you have available.

lhg posted 05-11-2001 01:26 PM ET (US)     Profile for lhg    
Of interest is the fact that zinc anodes are basically obsolete. All of the engine manufacturers are now using an aluminium alloy, which is considerably less "noble" than the old fashioned zinc. Mercury made this change about 12 years ago. None of their "branded" anodes are zinc any more. The Marine catalog houses are way out of date here still selling replacement "zincs". You can tell by the weight. The aluminum ones are much lighter.

Lately, Magnesium fresh water anodes have shown up. Magnesium is at the top of the "least noble" (meaning most sacrificial) list. These shouldn't be used in salt as they would go really fast. I'm now using these (in fresh water applications), and seem to work very well.

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