Sail drive paint flaking

[Knot Safety]

Good day folks

Question for all of you, but some background first

I have two Yanmar SD 50’s on my catamaran. They were repainted with sail drive antifoul paint in October of 2019. New anodes were installed at the same time.

About a month ago I noticed a paint chip missing on the starboard saildrive. Today I noticed a number more chips missing and a fair bit of crazing on the saildrive paint. About 15% of the saildrive is now exposed metal

The port sail drive has also now developed two paint chips.

The anodes look perfect. There is some growth on them but otherwise in great shape.

There does not appear to be any visible corrosion and there is no pitting that I can see or feel in the bare metal.

We will be hauling for the season in May.

The questions - anyone had this before? Idea as to cause? The fact that it is flaking to bare metal makes me think it’s an underlying paint issue as opposed to a antifoul issue. Is this a crisis that I should be hauling tomorrow to resolve or can it wait until May. The bare metal has me concerned but I am hoping it will just be a spot for growth instead of corrosion.

Look forward to your wise responses. My google work has not found this reported before. That most likely means my google use sucks

[HeinSdL]

I waited a bit to answer in case others had more concrete advice but seeing it's a bit quiet I can only confirm that I have similar issues with my SD40, paint coming off, touching up, coming off again, etc. I am not overly concerned, the aluminium is fine, not pitted, a bit like yours.

I read a bit about priming and painting saildrives and it would appear it's not trivial to get a good bond on the aluminium. The most compelling idea/technique I came across is to wet sand the aluminium while it is getting covered in primer (ie. use the primer as your sanding medium). In that way you remove the layer of oxide and don't allow any to return since it's getting covered right away with the primer. But I did not do that. I just now disassembled the saildrive and the tail is getting painted by someone in the shipyard. No idea what/how they are doing to it though, except that I trust they roughly know what they're doing. I suspect the paint on your drives hasn't sufficiently bonded with the aluminium.

My understanding is that a long as your anode is fine the aluminium is protected, but the more aluminium shows, the faster the anode will corrode. So I would occasionally check the anodes between now and May. I don't know that much about checking for stray currents or how other boats on shore power in the marina might effect your saildrives. If you changed your setup in Oct 2019 in addition to the paint job, then that might also be something to think about.

[a64pilot]

I would try alodineing the bare metal. On aircraft alodine is used both as a corrosion presentative but also as an excellent paint prep for adhesion.

[Knot Safety]

HeinSDL - thanks for the response. Glad to hear I am not the only one going through this. Nothing has changed other than antifoul paint and new annodes. I will continue to monitor this season and evaluate at haulout what the next steps will be

[Yoty]

Aluminium surface oxidizes in a millionth of a second. Sand, using your primer paint to re-etch the surface and ensure a 100% bond.

[Michael Pope]

The rule we used to use was twenty minutes . If you have any exposed aluminium showing twenty minutes after you .start Start again! If it is large or time consuming to sand or clean tape off areas and do it piece by piece.
The adoline or andoline aircraft primers work well . Bout I would always use an epoxy high build primer after it and seal with Linear Polyurethane finish coat . Three coat process is more leak proof on the Nano scale so it lasts a lot longer . And repair any nicks right away. It can last ten years if you are lucky. Mike Pope

[Library70]

Just to ease some of your fears: my sd50's have been bare aluminum since I bought the boat six years ago. I use a nonmetallic ablative antifouling paint applied directly to the aluminum. There isn't any pitting of the metal and the sink anodes last about a year and a half before they're fifty percent gone.

[Knot Safety]

That is great news Library 70. Thank you. I wonder if the previous owner has the same thing happen and all his paint flaked off. Suspect that is how mine will end up - bare aluminum