I have my Zunow and a Gazelle Opafiets and both are rust buckets. Finding Oxalic Acid here in Cambodia is hard or expensive.
So is there an alternative for rust removal other than purely physical, either sanding or wire brushing.

The Zunow
IMG_20180912_103646053_HDR by Bwilli88, on Flickr

For rust on chrome, try aluminum foil. https://www.google.com/search?q=alum...hrome&ie=UTF-8

I've had good results soaking small parts in white vinegar.

I use coarser grade of steel wool.

Barkeeper's Friend is primarily made from oxalic acid.

I thought rubbing with aluminum foil just deposits a bit of aluminum on the rusty surface and doesn't really do anything to remove the rust.

Wood bleach? Teak cleaner?
If you can find them, both are mostly oxalic acid.
OR -
How about electrolysis ?
The frame submerged in a solution of water / washing soda, an old car battery, a battery charger, some copper wire, a cathode (steel plate, rebar, etc), and Bob's yer uncle:

Rust removal by electrolysis

https://www.garagejournal.com/forum/...d.php?t=154196

My understanding is, the crinkled aluminum is a mild abrasive.

In Cambodia? Probably no more common rust remover than phosphoric acid. If you can't find it look for the bottle that says "Coca-cola"

A 24 hour soak cures a lot. Then a wire brush and retreat if necessary.

WD40 and really fine steel wool 000.
Go down to the river and find a boatyard and ask the local wags what they use.

Id be afraid the electrolysis would damage the chrome finish also like it could do to paint or clear coat.

Yes - Electrolysis will definitely remove paint
However, the bike he's talking about, a really beat Zunow, is way past the paint saving stage, so electrolytic de-rusting might actually be a plus, if it removed the rest of the paint on that bike.

If the paint is a concern, then OA is the way to go - Unless it's RED paint, which will fade ........... DAMHIKT ..........

I have a 220volt/12volt battery charger. It works real well as an electrolytic rust remover. I took baking soda and baked it at 300 degrees for an hour which changes it from Bicarbonate of soda to sodium carbonate or washing soda. That gave a better bath and it worked very well. 20 minmutes and most of the rust was gone.

I changed the solution to a large round tub to do a couple of forks and some rack parts. more pictures coming. It was super easy.

That is most excellent !
I've used electrolysis to de-rust all kinds of crusty machinery.
When you first see it done, it seems like magic

Been de-rusting parts for years, using aluminum foil, and I cannot complain one single bit about the results. True, if there is pitting, aluminum foil will not work to eliminate the pits, but surface oxidation is GONE! And, I keep the cleaned chrome waxed, using Mother's Cleaning wax and the chrome remains bright and shiny as time passes - so far and I have been using foil for more than a decade, perhaps two.

Also, I try to make my hobby of street restoring vintage bicycles, as green as possible. I will do my best to avoid chemicals when doing my vintage bike thing.

I've heard brass wool is better than steel wool? Haven't tried it myself yet.

I've heard brass wool is better than steel wool? Haven't tried it myself yet.

+1 Aluminum foil. My experience with steel wool on chrome was that it left tiny scratches that I was never able to remove making future rust harder to fight.

Interesting. It's as common as dirt here in Mexico. It is used to polish the ubiquitous cement tile floors.

So, this magic works. I am so impressed. I just need to dry the piece and take a hard brush and the loose paint comes right off. If you want to save paint you might want to try OA instead.

Before the rack

IMG_20170721_174123307 by Bwilli88, on Flickr

After the rack and seat stay

IMG_20181204_152549778 by Bwilli88, on Flickr

The "rusty" spots are really the bare metal showing through.

you can soak paper towels in vinegar, if you dont have a way to soak them, wrap the paper towel around parts you wish to derust. not sure how well it would work on really rusty parts but light rust works well.