View Full Version : sealing struts and rudders before bottom paint

07-01-2014, 12:38 PM
The parts are bronze, old (1970) and in good condition.

Rudder zincs always on haulout have mostly disintegrated, also the shaft zincs are usually gone or just barely there.
This haulout boat had been 4.5 years in the Chesapeake on the Back River near LAFB.

In the interest of getting more life from a zinc, what about sealing the bronze surface?

What I do not know is how long the zincs last before becoming non functional. Will sealing the metal surfaces improve zinc lifespan?

I had been thinking of smearing on a layer of polyurethane type rubber before painting on bottom paint.
I thought about coating the shaft but if you had to remove it, that would make it pretty hard to clean off a durable coating.

What about simply primer spray coating the prop shafts before bottom painting? will a primer seal the water out?

07-01-2014, 12:45 PM
The less exposed metal you have underwater , the longer your anodes will last.

copper antifoul is also aggressive on anodes.

Metal objects near your boat erode anodes

Loss of 50 percent mass is the normal replacement schedule.

4.5 years is a good run for anodes.

are these anodes zinc or aluminium ? Fresh water or salt ?

07-01-2014, 01:09 PM
zinc, brackish water.

07-01-2014, 01:33 PM
Brief technical overview of anodes


07-01-2014, 01:37 PM
The mass of zinc needed to produce the required voltage can be measured. If the mass become too small your underwater metals are no longer protected.

If you want to get scientific borrow a silver reference cell and measure the voltage field you create around the boat.

07-01-2014, 02:35 PM
Ok, but not an answer to my question of sealing the metal.
Consider the black PL, use a notched trowel to lay beads, let them set and go over with smooth trowel, then paint with bottom paint.
Idea being they become more electrically isolated. should be easy to do, question is it worth the bother?

07-01-2014, 02:43 PM
I'm sailing an Aluminium boat.

I have no exposed metal underwater...none. Shaft, prop, hull are all dry ..behind a barrier coat of epoxy.

I have. No idea what paint system you are talking about.

07-01-2014, 02:48 PM
Black PL polyurethane roof and flashing sealant. Can buy at Home Depot.
I used some on the hull underwater and it stayed in place, no problems after 4.5 years and stood up to the pressure washer.
It is waterproof and sticks to metals and wood and sanded Permaflex.
It also stuck to non sanded Permaflex, just not as well. Nothing sticks well to non sanded permaflex, not even Permaflex.
If you use it for another coat, sand the surface first or you can have interlayer peeling, my experience...

I mean the black PL stuck well enough you cant peel it off by hand. But if you use a orbital sander at 40 grit and high pressure, it may peel loose or it may sand down smoothly flat.

wizbang 13
07-01-2014, 02:57 PM
Wood boat?
Fastening material?
I slather my underwater galv rudder fittings with epoxy to reduce corrosion potential.
the idea is to reduce corrosion, not extend the life of the anode?

07-01-2014, 03:22 PM
yes, idea is it will do both, make the metal last longer and extend zinc life.
It is a wood boat
Fasteners are all silicon bronze, plank screws and bolts.
Shaft is some kind of stainless.
I think slathering with polyurethane, if it sticks and stays on the metal will work as well as epoxy.

07-01-2014, 05:43 PM
Sounds like you're really set on polyurethane.

Phil Y
07-01-2014, 05:50 PM
Try it, pop it back in the water for another 4.5 years, haul her out and report back. What's to lose? A couple of zincs? A few loose screws?