In 4130 I mostly use 028 (0.7mm wall). I’d use thinner if I could buy it, my first gen Tubus Fly racks are made with 0.5mm wall thickness.
4130 rusts very slowly and I think it’s a better option since it’s easier to braze, stronger, costs less, and builds a lighter rack. I have decade old 4130 racks on regularly ridden bikes that I haven’t bothered to paint.
Yes for every practical reason I should use chromoly
But I want shiny 
for vintage bikes with polished silver parts
I have decade old 4130 racks on regularly ridden bikes that I haven’t bothered to paint.
That’s interesting! But where do you live? I would be a bit worried about rust here in the rather wet UK, where they also put salt on the roads in the winter.
Last I heard, Alex was still in Seattle, maybe not as wet as some parts of the UK but sort of famously the dampest corner of the USA. That’s not actually true (not even close), but it doesn’t have to be true for it to be famous…
At least they don’t salt the roads here, or not very often since it almost never snows here.
I live a mile from Alex, and I have an 11 year old CrMo rack I never bothered painting. Sure it’s got rust, but not even the hint of a beginning of a structural problem. No pitting, that the eye can see anyway, so far. I could steel-wool it and lightly oil it and it’d look new — but I don’t, because it was made for a utility bike where I don’t care about looks. It even makes it less likely to be a target for meth-heads, so, bonus.
I know that’s not for everyone. Racks that I’ve made for customers were always painted (or PC), or plated, or SS. I’m not advocating for letting stuff rust, just saying it’s more a looks thing than any real problem. Chrome-plated racks probably get more pitting and rust-through, I assume from not rinsing or neutralizing the plating solutions enough (not a plating expert).
A rusty rack can leave a stain on clothes or anything else the rack rubs against, and some people won’t like that. Me, I just always wear rust-colored clothes. 
They might have started out white but got that color from riding and camping in red-rock canyons in for example Utah, where I tend to go whenever I’m not in Seattle. Didja know those rocks are red from iron oxide? They’re literally rusty. So rust makes me feel like I’m on vacation.
Yup, I still live in Seattle and my bikes are stored inside. I really only get surface rust at the joints, the parts of the tubing with black mill finish rarely rust.
Chrome is really shiny.
You should make a small/simple rack with both and see which one you prefer working with.
Who controls the features of this board? We need a ‘LOL’ reaction button.
Yup, I still live in Seattle and my bikes are stored inside. I really only get surface rust at the joints, the parts of the tubing with black mill finish rarely rust.
That’s interesting. Seattle is probably even wetter than UK! I’m not sure what the black finish on such thin wall tubing is– probably not really mill scale, but the remains of some lubricant used when drawing it or something perhaps. But you can create a sort of mill-scale layer by using “gun blue”, which looks nice, and is fairly durable. I’ve done this on a couple of “raw finish” frames (and I put clear-coat on top, but that would soon get knocked off a rack in the places where the bags are attaching and rubbing).
Seattle doesn’t make the top-10 list for inches of precip. Miami Florida and New Orleans get literally twice as much rain as Seattle. All the top 10 are in the south. People don’t seem to think of Miami for rain, but it’s at or near the top of most lists
On the list of cities with the highest number of days with measureable precip, Seattle usually ranks about #8, depending on methodology. Most of the higher ones in the northeast, especially upstate New York (not the city), Ohio or Pennsylvania (Great Lakes effect).
We do have a lot of gray days in winter, but we pretty reliably get a drought in summer. Come here in August if you have a choice. Now, the west slopes of the Olympic mountains to the west of us, that’s an actual rainforest. Rain is measured in feet there. It looks close to Seattle on a US map, but the difference in ecology is stark. We’re in a ‘rain shadow’ from the Olympics.
But to bring some actual on-topic content: Much of my shop is outside, under an awning, due to my basement being tiny. Any uncoated steel or iron out there gets sprayed with one of those rust-inhibiting goops, like FluidFilm, about once a year in Fall, mostly makes it through the year. Though not without some rust. Like I said, I don’t worry about rust as much as most people.
What do you-all like for rust preventative on machinery? People with climate-controlled shops need not answer, ‘cuz I hate you. Lookin at you, Duane!
Here’s most of what I’ve tried:
Just emptied my can of FluidFilm so I will be looking for what to buy next. I don’t want something that hardens like linseed oil, that can interfere with moving parts and throw off measurements, like a height gauge on a cast iron surface plate. (I have to wipe off the FluidFilm for use, re-apply afterward)