Painting Inside of Hull on Steel Sailboat

Remove any loose stuff and treat the remainder with OSPHO or similar then begin your paint system

Hello ,
i'm hearing rumblings and mumbling from steel hull folk about the viability of painting bilges while the boat is still in the water. The claims being made are that painting below the water line shouldnt be tried unless the boat is on the hard as the difference in temperature will prevent the paint from sticking ( no matter what is used)
has anyone got experience of comments on this either way?
thanks
marc

Makes sense if you are up there in cold water since epoxy is exothermic. Anything below 50 degrees is a no-go for most epoxies. The hardener won't "kick" and when it does it won't fully harden. I'm not a cold water guy, but I'd heed the rumblings and mumblings.

Example: http://www.westsystem.com/ss/cold-temperature-bonding/

When concrete ballast gets wet, the lime in the concrete reacts negatively with the steel and will corrode it from the inside. Big problem as you can not see it. Have to ultrasonically gauge the hull in the ballast area if you have any questions that the concrete's integrity is in question. Have to be sure that the concrete ballast is stone dry, no pun intended.

A while ago, I recall seeing an epoxy product called Red Hand used to prime steel surfaces / welds in the bilges prior to painting: I think it has been renamed since to Intergard-822.

http://www.international-marine.com/products/info/intergard-822.aspx

right - but if a person waits for warmer weather/water... up closer to the operating temp of the epoxy (entirely possible late August) i would expect that a 2pack paint would kick even if the ambient temp is much higher than the water temperature.
or is that an incorrect assumption?
marc

just curious, isn't rebar encapsulated in concrete a common construction technique?

what prevents the rebar from corroding and weakening the structure?

You probably can't see it because you don't have years to wait for anything you can see.

http://www.cement.org/docs/default-...causes-of-concrete-deterioration.pdf?sfvrsn=4


"Concrete and the Passivating Layer:

Although steel’s natural tendency is to undergo corrosion reactions, the alkaline environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At the high pH, a thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does not actually stop corrosion; it reduces the corrosion rate to an insignificant level. For steel in concrete, the passive corrosion rate is typically 0.1 μm per year. Without the passive film, the steel would corrode at rates at least
1,000 times higher (ACI 222 2001)."

It is common practice to use concrete for fixed ballast. Coal tar epoxy is a better coating for the hull area where the concrete is poured than the epoxy paint yachties use because it doesn't chip or crack. Cement coating of potable water tanks is also used to counter acidity and prevent the steel from rusting.

Keeping rebar away from an external water or electrical source is what makes it safe as a concrete reinforcement for structural applications. In the 90's I was taking some night classes at UC Irvine, and they had an entire 7 story, brand new building condemned before it opened, as the rebar was exposed to the environment, corroded and then crumbled the adjacent concrete. Lessen learned for the county, at a cots of millions. Similar episodes have happened in CA for new bridge construction, failures before they even opened, cheap contractors with unskilled labor or willing to take short cuts, with poor government inspection / oversight.

For concrete ballast, keep it 100% dry / sealed. Not much else to say about.

Ferro-cement boats disintegrated from the inside out due to their (chicken) wire reinforcements being exposed to seawater / and stray currents. I am sure this forum has already addressed that topic.

Yes, concrete is a fine ballast material when used correctly. It requires the right coating / sealant of the metal hull as well as eliminating prolonged exposure to bilge gunk. If exposed to a seawater source it will create the reaction I stated over time. How fast will depend on many variables.

If you are unsure of how the concrete is put in place, better to resolve the issue. OP did not state how old / who built the vessel.

My son tells me that in concrete poured buildings the re bar is very often epoxy coated particularly for areas that will be prone to dampness.