Mast paint yes/no?

[Icarus]

Originally Posted by jtsailjt

I have a friend who just bought a boat with an aluminum painted mast. There is some bubbling of paint around some of the fittings but overall the mast looks great and the step and bottom of the mast are fine. The quote he got to remove all the mast hardware and repaint is a good deal higher than he wants to spend. I've never liked the idea of painting aluminum because it always seems to have problems with adhesion or eventually water gets behind it in places (around fittings) and is held in place by the paint so corrosion/bubbling occurs.

So, would it be acceptable for him to have his mast sand blasted with the hardware in place and just leave it as bare aluminum or would doing that invite further corrosion? Does aluminum need to be protected by anodizing or painting or is it OK to leave it bare? I've always had masts that were bare or anodized aluminum and have never had any problems with them. Other than for aesthetics, is there any reason to paint an aluminum mast?

Just pull off the fittings, if they are made of stainless its a good idea to isolate them from the alu mast.
Sand the mast, etch prime and spray paint with acrylic polyurethane.
Mine has been painted 15 years ago and is still in good condition.
Or leave the thing bare works as well, just keep in mind the alu used in mast manufacturing is not a marine grade.
Either way is ok

[Yoty]

A sanded aluminium surface re-oxidises in a millionth of a second.
When glueing or painting, sand it on to the Al surface.

[ThoFre97]

Originally Posted by rbk

The anodizing creates an oxide layer almost as hard as stainless steel. I would assume it also makes it brittle and prone to cracking even if just at the surface = aircraft failure?

Regarding Brittle Surface Layers:
Yes, the thin layer of anodize is extremely hard and sometimes serves as a fatigue crack initiation site. The anodize layer has an equivalent Rockwell C-scale hardness of around 90; the hardest stainless (440-C) is around 62. Typical annealed 300-series stainless barely registers on C-scale at maybe 20.


I was an engineer at a company that made hydraulic actuation components (mainly electrohydraulic servo valves). Their main product line used a 2024 aluminum housing with sulfuric acid anodize, dyed black. Over the years, working pressure increased and fatigue crack failures started occurring. Engineering looked into it and discovered that sulfuric acid anodizing on 2024 reduced the fatigue life by ~75% (IIRC) relative to bare 2024. Chromic acid anodize on 2024 had a far lesser impact but I don't recall it quantitatively.They changed over to 7075 with a chromic acid anodize, undyed (silver) that had a fatigue life that was at least three (IIRC) times longer than what they started with.


The one key lesson I have retained is to never use sulfuric anodize process on 2024.

[ThoFre97]

Originally Posted by a64pilot

...
Anodizing is best, but alodine is a pretty good prep and corrosion protection for aluminum that’s painted.
I’d at least alodine it if I were to paint.
...

Yes on Alodine or other brand of chromate conversion coating. Actually, these days there are HCF (Hexavalent Chromium Free) versions. Nice background here: https://www.besttechnologyinc.com/su...rsion-coating/


Going with bare aluminum seems rather risky to me. What about going with a chromate conversion coating alone.

[Don C L]

Welcome aboard ThoFre97!