I have a 55 year old cast iron drain system.

One of the fittings is weeping rusty stuff. There are no holes, broken pieces or large leaks rather the pinhole type that form rusty barnacles. If you wipe of the barnacles you would likely see moisture but if you don't touch em, there is no visible moisture.

Im sure the plumbers here are familiar with what I am talking about.

Should this be a concern for me? Is that indication that it needs replacement?

I had a plumber look at it in passing and he said don't sweat it that it could go on for years like that.

Any advice about this situation would be appreciated.

I wouldn't worry about a couple little spots on a vertical pipe, but on a horizontal run I would replace it if it was in the open, otherwise I would wait until it actively leaks. Those "barnacles" are corrosion from the inside out, so it is basically a hole with a little plug in it now. DO NOT BREAK IT OFF!

take a small metal object and tap the cast iron pipe. if it rings like fine china it is still OK with lots of life in it. if it thuds like kicking a rusty fender on a F450, replace it..

These barnacles are light and flaky....

First off I know the proper fix is replacement.

With that said....

If there are no visual leakage or cracks...can I get away with painting it?

the cast iron in my house was installed in the 50's. i have a couple of spots where barnacles are growing. i've been here since 84 and they are still there.

Packy- Perhaps that's a little consolation for me knowing even a plumber is willing to live with barnacles.

So I was going to spray paint some of the rusty pipes until my neighbor told me that spray paint would make the pipes corrode faster. Any truth to that or was the neighbor just drunk?

why would paint desolve cast iron pipes? maybe in CA where everything causes cancer would you find such an absurd claim.

I can understand the reasoning to some extent, same as an old bed liner in a pickup. Moisture can still get underneath and cause corrosion and doesn't dry as fast as open air exposure.. That being said, it is incorrect since the corrosion starts on the inside, so the outside is irrelevant anyhow. Paint it and put a bow on it if you like.

At what point do you guys suggest to your customers to replace sections of cast?

In this case there is no dripping water or visible cracks or holes but yet its obviously weeping a bit indicated by streaking rust stains or the barnacles.

Does someone wait until you cant use the drain anymore?
Is a repair like this more $$ if you had to have someone out the same or next day like a service call rather than something you would get an estimate for?

Can you take a picture of the area that it is located in and post it? Its physical placement in the system and other factors such as lack of strapping above it can affect the urgency of repair. I would get estimates in any case, but we may be able to guide you more accurately with a picture.

as I said above, I 'ring' the pies with a metal object. if the pipe rings a high pitch noise they are fine. but if they 'thud' like a F450 rusty fender, it is time to replace.