MoreCarbs

Just picked up a 90's Specialized Rock Hopper, but it's got quite a few areas of scratches under the paint and primer leading to rust.

Is there a certain bristle material to use by hand or in a drill to knock off the rust while preferable keeping the paint mostly in intact? I'll plan to use clear coat rattle can (or nail polish) on the bare metal after the rust is off.

jccaclimber

IME you’ll have to strip any rusted areas to clean bare metal, at least locally. I’ve had success with a wire wheel, though careful use of sandpaper works as well. Obviously a gentle touch is appropriate, along with proper PPE if using a wire wheel or other powered rotary tool.

stevel610 

Try balled up aluminum foil.

Crankycrank

These work great for small chips and scratches. ProMotorCar PrepPen Adjustable Sanding Pen 3437 | O'Reilly Auto Parts (oreillyauto.com) Can be found in various sized tips for very narrow to wider paint area removal. Just a cluster of very fine glass strands that give you very good control and rapid removal of paint/rust leaving a surface ready for paint or primer. If you have a small chip you can just twist the pen leaving a very small round prepped surface as opposed to sandpaper which creates a larger area. Make sure to wear rubber gloves and cover the floor as the glass strands break off during use and easily get in your skin and near impossible to find and remove. Auto parts places and Ebay, etc. have them for sale.

WizardOfBoz

I would suggest that anything that doesn't remove some local paint, to get down to the bare metal in the scratch, will not be effective long-term - you'll get rust coming up through the painted scratch. For this reason, I'd use sandpaper, maybe one of those tools that use a small thin loop of paper on a spring-loaded stylus. Like these.
Then prime and sand, then paint and sand, then clear-coat and sand and polish.

N2deep

If you're not going to re-paint the bike I suggest washing the bike with soft cotton cloths to preserve as much of the paint as possible. If you still wish to remove more of the rust try either the White and or Grey scotch brite pads as these are slightly abrasive and used to polish paint. Try the scotch brite pads under the bottom bracket to see if they remove too much paint. Best wishes.

dedhed

evaporust

​​​​​​https://www.evapo-rust.com/

MudPie

My first idea is to use a fine, wet sandpaper, and favoring light strokes with many passes. With sandpaper, you can feather and blend the surrounding paint so there is a smooth transition from the original paint to the sanded area. Smooth transitions help with paint adhesion.

A potential problem with a wire brush is it is likely to damage the surrounding paint with visible scratches from the wire.

3alarmer 

...unfortunately, what often happens is that the rust blooms along under the paint surface, along the face of the steel. You can't really see it, but by preserving the existing paint, you effectively maintain it. I wish I could show you some photos of frames I've stripped, because the webbing as the rust expands along underneath the paint is quite interesting. If you don't sand or use a wire brush to remove the existing rust that you cannot see, under the paint, it will eventually reoccur. And rust never sleeps.

MoreCarbs

Picked up a bottle of evapo rust to try first after cleaning the frame.
If that doesn’t work, then I’ll try foil maybe with vinegar.
if that doesn’t work, I’ll try wet sandpapering.
if that doesn’t work, I’ll try a hand brass brush.
if that doesn’t work, I’ll try a rotary brass brush.
if that doesn’t work, I’ll try a rotary stainless brush.

fingers crossed

saw a guy on YouTube try automotive solutions like clay bar and rubbing compound on random orbital buffer to good effect, but wasn’t sure which step/product actually got the rust off.

capnjonny 

I find that there are 2 kinds of rust on a bike, minor and major. Minor includes the light rust you might find in scratches in the paint . Major rust is where an area has a thick crust of rust that has been building up for some time.
For the this crusty rust the best thing I have found is to scrape the area with a disposable knife, the kind that you snap off the end of the sliding blade to renew the sharp point and edge. Using a steel brush will just not penetrate the crust. The knife can be used fairly surgically just on the crusty area.

After the crusty areas are scraped down to bare metal , coat the entire frame with naval jelly , let work for 1/2 an hour then rinse off. Persistent areas can be treated again with naval jelly and you can scrub the jelly into the rust with a stiff tooth brush or similar brush with stiff nylon bristles .

Before you coat the painted frame with jelly first try it out on an inconspicuous place to be sure it doesn't react to the paint.

I have done this on an old Raleigh sport with lots of rusty scratches and the improvement was considerable.

The areas that you scraped and treated with naval jelly should be immediately primed with an etching rust preventative primer as soon as it has been cleaned and dried.

After treating the frame rinse with water then wash with Dawn dish soap and water, rinse, then wipe down with alcohol to remove any remaining surface contaminants being careful not to wipe any decals or stickers . Alcohol or acetone can wipe the ink off them.