Physics Dept. needs Waterproof, Black Coating for Brass

Hey y'all...

I'm a graduate student in Physics at Georgia Tech and I am having some trouble finding the appropriate coating for my application. Basically, I have a big brass cylinder (41" long by 5" across) that is supposed to be permanently submerged in distilled water. We chose brass for corrosion resistance. We can spin the cylinder around and then put little white particles in the water to visualize the flow. I need the cylinder to be black to maximize the contrast between the particles and the cylinder. I also need the coating to be somewhat mechanically resistant as there is a plastic ring that slide up and down the cylinder. I have tried the following treatments with little success:

1.) Prime and paint with regular outdoor paint. This failed miserably since the primer sucked water in under the paint and it puffed up everywhere.

2.) Regular paint without primer. This worked a little bit better, but eventually the water got under it anyway and puffed up the paint. Wherever the paint puffed up the plastic ring took of the paint as it slipped by.

I realize in retrospect that these weren't the best ideas, so I've been looking around to see what other options I have. I've come up with the following:

1.) Marine Epoxy-based paint. I don't particularly like this one because I don't have a ⇦ on eBay or Amazon [affil link] and I need as smooth a finish as I can get.

2.) Powder coating. The guy at my location coating shop says it should be ok when submerged in water, but he wasn't 100% sure.

3.) Black Chrome? I'm not even sure if this is possible for brass and I don't know how black you can actually get it. Also I'm not sure what the water resistance on it is.

What do you guys think would give the best results? Remember the criteria are that it will be semi-permanently submerged in water, it needs to be black (flat black is best but not critical), it needs to adhere to brass, and it needs to resist a little mechanical wear. I appreciate your help and am definitely open to any other ideas that you experts come up with.

Best Regards!

Black nickel is more of a dark gray and is sometimes mottled.
Black chrome would be a good choice as it is a black color, fairly uniform and somewhat durable when compared to a plastic ring.
It is a commercial application! there are not many shops that do it, so the price may be a bit steep.

On a student budget, I would contact every solar collector manufacturer that uses metal collectors. You might be able to get "drops" (leftovers) for a cheap price. They are black chrome over brass.

Why not a tight sleeve made of a hard black plastic known as Nylamid. Alternatively, you may thermospray it with chromium oxide (either ground or as is). More expensive but perhaps within budget.