Pulled the fender off my cross check today and found this. Anything I can do about it? Is the frame totally screwed?

I would apply a rust converter, a Web search will yield many suggestions, which will turn the rust into a stable substance which will act a a primer, then paint over the area to inhibit further rust. Get the converter into the mounting holes and frame tubes as well as you can. I doubt the frame is seriously compromised but frame experts on the forum may chime in.

I am not a frame or metal expert, but, I agree with dsbrantjr's assessment. I use a spray made by Lock-tite, Extend Rust Neutralizer. In my experience, it works well.

will spraying that into the tubes damage the bb at all?

Frame is toast, what size is it???

Really it's just aging. Best- Scrape/sand off all rust you can, and prime/paint with model/touch up paint. Pull off all components and apply rust converter (I use https://www.evaporust.com/ ) internally and then rust inhibitor (Frame Saver). Andy

Do more then just spray over the rust. Use mechanical abrasion to remove what comes off easily. A 3M pad can work. A good rust remover that is gentle is "Bar Keeper's Freind" it is a powered cleaner for use in the kitchen, kind of like Comet but it is acidic and removes corrosion. Some of that on a scrubby sponge will remove light surface rust. It costs about $1.50 per can. It is abrasive enough to dull gloss paint so watch how you use it. It works best on stainless steel cookware but also on most matal bike parts. Make a paste and use it with a toothbrush to remove light oxide on aluminum too. Or as a chrome polish.

Then after removing what you can, rinse well, let it dry a good long time then use the converter, primer then paint. Paint is best applied with a brush because spray paint gets into everything

The frame is fine. That little bridge has very little structural value anyway, its main purpose is just as a fender mount. But as others have said you want to get some kind of schmoo down in there to try to stop it rusting any more.

Keep in mind, rust converters, like the Lock-tite product, state that rust must be present for them to work.

Ospho works. Is Phosphoric acid that chemically turns the rust to iron Phosphate. You can either paint over it or leave it.
https://www.worldpaintsupply.com/osp...tal-treatment/

Thanks all. Got some Loctite naval jelly. What should I use for paint after I’ve applied the Loctite?

Ideally, an etching or bare metal primer, then your enamel or lacquer of choice.

Keep in mind that the naval jelly can form phosphate crystals, which are grayish-white and powdery, on the treated steel. Ospho recommends wiping with mineral spirits to remove them before painting.

Assume any bare steel has an oxide layer after exposure to oxygen. It may not be visible, but it's there.

(spanish native lang here)
I see in the pic that its over the metal, isnt corroding the metal, you can also repaint your frame, use premier paint and then the desired color, you can use a drill with a cable ball thing adapter (i dont know how is named in english) and you can strip all out paint and specially oxide parts and you will see that the oxide is over the metal layer, the issue i dont like to use much this method is that let some micro scratches on the metal, no big deal but i dont like it

Totally screwed? Nope. You're just getting started!

As others have said, that rust is absolutely minimal, that part is not structural, and even if it were, you can see from your picture that it still have loads of wall thickness left.

It may be a bit jarring when you have a newish bike to see a patch of rust, but rust moves very slowly and it would take years and years of very abusive conditions to get to the point where the frame is compromised. If youa re using it as a winter bike in a place where the roads are salted then this might be 5 years of solid Winter use with no maintenance. Probably more than 5 years, but at least 5 years. If you live in a place where the roads are not salted, expect decades of use before rust becomes an issue. It is far more likely that every part of the bike wears out, or the bike is totalled by 'misadventure', before there is a problem with the frame due to rust.

If I were you I would sand the surface rust off as much as is easy to get at, then use nail polish to cover the bare patch, then remount your fenders to cover the nail polish.

Best practice, apply a rust converter/bare metal primer, then the nail polish. Ospho or Corroseal will stop the rusting process and provide a base that the steel won't rust through. Ospho has no pigment, but it will turn red rust black, while Corroseal seems to turn the whole thing black and works like a traditional primer.