A large part of being smart in any area of expertise comes from doing your homework. And this is especially true with hydraulics. Here's a story from one of our readers which illustrates this point pretty well:
"I'm fairly new to hydraulics and I thought coating the interior of a hydraulic tank I'd made with a zinc spray (cold galvanizing in a spray can, like paint) would be a good idea. The idea was fine but the reality was not so good. I had no way of predicting that the components of the oil would dissolve the zinc coating... I'm fortunate that there is little I can do at this point that would further endanger my soul."
Call me superstitious, but I'm not a fan of painting the inside of hydraulic tanks, period. To digress for a moment, it reminds of a client a few years back who had a machine with a history of the oil to water heat exchanger failing and contaminating the hydraulic system with water. He asked me what he could do about the problem. When I suggested switching to an oil to air heat exchanger, there was a long pause on the other end of the phone. Followed by: "Why didn't I think of that!"
Here's the thing: engineer out potential problems if you can. If you don't use an oil to water heat exchanger, cooling water can never contaminate the oil. If you don't install pump intake isolation valves, the pump will never be started with the valve closed. And if you don't paint the inside of hydraulic tanks, the paint can never come off.
Oh sure, all of these thing can be done in such a way that the risk of the worst case scenario happening is minimized. You can specify a double-walled oil to water heat exchanger. You can install proximity switches and interlocks on pump intake isolation valves. And there are paints specially formulated to withstand contact with the myriad of different hydraulic oils out there. Although as far as I'm aware they don't come with a guarantee. Like I said, the worst case scenario is minimized, but NOT completely eliminated. The risk remains.
And why should you need to paint the inside of a hydraulic tank anyway? Think about it for a moment. How many other hydraulic components are painted internally? None. Zilch. Nada. So why don't these components rust? Because the hydraulic oil is formulated with rust inhibitors.
But the hydraulic tank is different, right? I mean water settles to the bottom and air circulates in the top. Well, make sure the tank has a drain point - and see that it's used. And if you install a desiccant breather, rust on the inside of the tank's air space will be minimized, if not eliminated.
And so what if a bit of light surface rust forms on the inside of the tank anyway? Aside from not looking real pretty how bad can it be? It will likely make a small contribution to contamination load, but the system's filters will soon take care of this.
I've designed and built hundreds of hydraulic power units. And never painted the inside of a single tank. In some instances, I've actively discouraged it. I've also worked on many hundreds more built by others. And I'd guesstimate less than two percent of those had the inside of their tanks painted - some successfully and some not.
Either way, I'm yet to see any evidence that leaving the inside of a hydraulic tank unpainted is detrimental to the system. But as this hydraulics user found out the hard way, painting it can be. Especially if you don't do your homework first!
P.S. on the subject of homework, "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" is essential reading. And it's available for FREE download here.