Is rust inside the tubes of a 30 year old steel bike always a bad situation?

I have a mid-80s steel frame (Sannino) that I've had for a couple of years. I recently changed the crank and bottom bracket so while it was open, I was poking around with my fingers and shining a flashlight in there to figure out if it was Columbus SL or SLX just out of curiosity*

I live in a dry climate and never ride this bike in the rain unless I get caught out in it (which rarely happens because I truly hate riding in the rain and will be careful to avoid it). The bike lives like a bat hanging from the ceiling of my garage when not in use. The garage is cool and dry.

The rust is a good layer of dry stuff, but doesn't appear too deep. For example, it doesn't obscure the fairly subtle SLX tube's rifling. The frame itself is in very good condition - original paint and chrome in good shape with no surface rust to speak of, no bubbling, and just some nicks in the paint from normal use which I've touched up with Testor's paint, invisible from 4 feet away.

Any advice whether I should treat the inside of the frame or just be content that it's in good shape for a 30 year old bike and will likely last another 30 in a dry climate

*re: the tubing. An old catalog I saw online said SL so that's what I thought it was. My bike doesn't have original decals and I'm trying to get some good reproductions made, and wanted to get the correct Columbus decals. The frame pictured in the catalog wasn't exactly like mine (painted fork instead of chrome), but whatever. I recently saw a bike for sale online that was identical to mine (except larger) - had the same chrome parts- and it had original decals, one of which was an SLX sticker.

In checking my frame, it's clear that the seat tube has the SLX rifling.The down tube is harder to ascertain because of the way the tubes come together in the BB cluster, I really can't get a good look - it's obscured by the ST. The chain stays have some sort of longitudinal ridges at the BB junction, but I'm not sure if that's a diagnostic feature. I don't know if I'll bother removing the fork to check the DT at the HT, because I think the rifling of the ST at the BB makes the diagnosis SLX. The top of the ST where the seat post comes in does not have the rifling, but that is typical of SLX. From what I've read, it probably doesn't really matter, both are great tubes, with the SLX maybe a little heavier, but a little stiffer (which doesn't matter to me because I'm old and weak). I'd just like to have authentic decals.

I got some good photos of that SLX bike and was able to get good decals made locally to replace the ones I had gotten from an online source that really weren't close at all. I also got some of the correct Columbus decals based on what that are on that bike.

Thanks. I'm pretty confident about the SLX tubing (because of the rifling in the seat tube) - but that was just a footnote on my concern about the rust inside the tubes.

this might be a good question to direct to the frame builders.

i had (have?) a custom steel frame that rusted completely through on the DT, ST, chain stays.
i think it was related to the original power coating - maybe not done completely correctly. so there was what looked like blistering of the powder coat. i was watching for a couple of years before i finally decided to get the frame re-coated.
when the coaters sand blasted the frame, a couple of spots when right through to holes. i probably had a compromised frame for a couple of years and was riding it plenty hard.

look for paint blistering. most likely you'll find holes in the frame before you're in any danger.

STEEL is just that good.

let me know if you'd like to see the photos.

I imagine some of the heavy hitters will be along to give you the facts. My understanding is that a light coating of internal rust is no problem at all and you can probably get away with doing nothing about it. I bought an older Romic that had some light rust along the seat tube. I sprayed it with WD40 and ran a rag through the tube a couple times wiping off as much of the rust as I could. I also sprayed it with Boeshield, and I've used that on my other bikes as part of tear down and rebuild. My other bikes, including a '78 Motobecane, had no or very little rust on the internal tube surfaces, which surprised me. I actually thought it was silly to spray Boeshield into the 37 year old Moto, since it has survived this long with no coating, but I did it anyway.

ps: Some folks also advocate coating the inside of the frame with boiled linseed oil. i did that with a recent '85 Nishiki after I powder coated it.
don't know if it'll work, but i think it smell good...

Por15 is a paint that's made to paint over rust and trap it from spreading. I have used it on car frames. I guess you could use a shotgun bore cleaner. Sounds hillbilly but I think it would work nice. Wear gloves, this paint does not clean off your hands, it's on there til it wears off, from experience of course.

Treat it with Framesaver or (if trying to pinch pennies) your choice of "lesser" concoctions like Boeshield, LPS #3 , Waxyoil or boiled linseed oil. I've used them all...it can't hurt!

I wonder how much that any leftover flux from brazing might be hygroscopic, thus attracting moisture from relatively dry air.

Perhaps this would need to be washed out before any coatings could be applied effectively.

I've worked on many bikes where a pile of loose rust had fallen down into the bb shell, even raced one of them for years, but have yet to see a frame failed from rust-out.
I should mention that I live in California though, where humidity is very low most of the time.
Bikes exposed to garage or outdoor freezing temperatures might be much more likely to suffer from condensation in the tubes.