Exhaust Manifolds

I pulled the exhaust manifolds off my boat yesterday to inspect them. After soaking them in vinegar I power washed them and they are in very good condition. I'm not sure of there age, but there was no heavy rust. What came out us mostly that black crud that always seems to plug the drain plugs. Tapping on them with a small hammer produced a very solid sound.
What would be the best thing to use to help prevent rust in the water jackets?
I'm going to paint the outside with a high temp paint.
I boat mostly in fresh water.

What engines do you have? Gas or diesel. Are the manifolds cast iron, fresh water or raw water cooled, and are they used in salt water?

Ted

To answer your questions, the manifolds are cast iron, raw water cooled and the engines are gas ones. Mostly used in fresh water.

As long as the manifolds are cooled by raw water, fresh or salt, they will be susceptible to some degree of corrosion. The only way to eliminate the danger of rust is to cool them with the engines coolant loop.
I have replumbed some Perkins motors to accomplish that, it worked out well at moderate throttle settings, but temps would creep up at WOT.
It’s dependant on the capacity of the heat exchanger and it’s piping.
You’d have to do some homework on your particular engines specifications to determine if they could safely handle the additional BTUs.