I would like to give brazing a try by building bike racks. I have done MIG welding but I'm clueless about brazing. On Mcmaster.com I found 5/16" stainless steel tubing (8989K46) and brazing white paste flux (7693A1). Could somebody please recommend me which rods to get? Also, would a propane torch be hot enough for this?

You ideally want a Propane/Oxygen mix. You can get the ones with the little oxygen bottle for around €120 not sure whats that is in US.
I have tried with just the Propane/Butane mix and it does not get hot enough, the oxygen really gives it a boost.
The other consideration is the flame size, those little brazing kits have really small flames so it is hard to get a big area hot.

Stainless is not an easy way to get a start. It doesn't tolerate overheating all that well, is hard to get up to temp and once you get it up to temp it's really easy to overheat it. You might want to consider going with 4130 and bronze brazing alloys as a cheaper alternative. However, you would definitely need an oxy/fuel torch for that.

If you were going to stick to silver and stainless:
You want 76975A64 filler

You probably want black flux 7645A41

I can't recommend starting with plumbing torches, but some people report success with MAPP and silver

To make racks MAPP-air will work even with brass. It works but it is far from ideal; you have little control on where the filler goes so building a nice fillet is difficult.

I learnt to braze doing exactly what you are proposing, using an ordinary domestic "Primus" propane torch. Get the right filler and flux and be prepared for a steep learning curve.

I used Cycle Designs fillers and their stainless white flux. As others have said, stainless is not very tolerant of errors, so be prepared to throw things out and start again if it goes tits up.

I'm currently living overseas and rather limited on my options for a heat source. Propane is the most readily available at a reasonable cost. I figure I could spend my money on a high end torch head to compensate somewhat. Speaking of which, any recommendations on that?

Is stainless all that more finicky? Should I start with straight steel and make a rack for my buddy before moving up to stainless for my own rack? If I stick with stainless, would I be better off with a .035 wall instead of the thinner .028?

On mcmaster.com stainless 5/16 steel tubing is a lot less expensive than plain. Am I missing something?

Also, I was looking at Nickel-silver, which has no silver but is called that for the color. How well would that work vs. a silver alloy? Anybody care to share the Mcmaster item number? That site can be a pain in the arse to search.

For making small joints as in racks stainless & silver is not that finicky but you will need practice for sure. Go for 0.035 wall; stainless (especially 304) is generally weaker than 4130 steel.

Nickel-Silver is a filler that melts at brass temperatures. To join with stainless you need a special flux. It can work with normal brass-brazing flux but it's much harder. Cycle design has some.

I haven't looked at McMaster Carr but Aircraft Spruce is a good source for steel; they have the basics for racks in stainless as well.

4130, and brass is so much simpler , ..

then you can get it powder coated for a durable finish.

Given stainless filler is common for TIG, maybe tooling up for that will be
an alternative.. for stainless tubing.

does mcmaster ship overseas? 4130 would be more expensive than a welded stainless. I would definitely go with .035 either way. McMaster sells bronze, but I don't think they sell nickel silver. In fact, I don't think they sell bronze flux, just flux coated rod.

Nickle silver is finicky, and using it on stainless is less than satisfying even if you know what you are doing. I would skip it.

I'm overseas on a military installation with access to USPS mail.

So far I'm going with the 5/16"OD .035" walled stainless steel rods, black flux, and silver filler. I'm also getting a Bernzomatic Trigger-Start Hose Torch, which I hope will burn hotter than a plain vanilla torch.

Thanks for the input.

4130 tubing , brass, and an oxy-propane set- up is the way to go.

Racks take a great deal more work than most people think, there is a lot of mitering and more joints in a rack than there is in a frame.

when I was in the Air Force, I did my brazing at the auto hobby shop

There was a recent sale of all the equipment of a dismantled AF base here in the Netherlands, and with the stuff there you could pretty much start your own bike factory! TIG, brazing torches, welding tables, it was all there.

I would go with the stainless tubing and some safety silv flux, and some safetysilv solder. I use the 45% stuff, and I think it is OK for what you are doing. I didn't have any problems doing stainless parts. Tigging stainless the first few outing was tough, but I didn't have any problem on my racks, or other fittings. You will find lots of it on ebay.

SS solves a lot of finishing problems. The powder coating can cost as much as the racks would cost to buy. It's worth it compared to custom racks, and to get what you want. But if you go stainless it costs about the same as 4130, and can be finished bright. It is not as strong as 4130, but some pros use it for racks, and the racks have normal dimensions and strutting. Another option is 4130 with some home paint, and stainless wear points. One home paint is appliance epoxy with the finish heated to about 160 to harden it off. White had an interesting rack that used 5/16 tubing, and 3/8 tubing where the bags go on, a short sleeve of it. That allows the 4130 to be painted with low wear enamels, as frames are painted. You can also do the eyelets an mud guard points in stainless.

Going straight propane is OK ish, better to go to two torches, than to go super fancy. For your rack the one torch should be fine. MAPP is a mixed blessing, it is slightly hotter, but it seems to contaminate more. Both MAPP and propane worked fine. At first when using MAPP I thought it was the solution, but later I returned to propane.

Propane is an entirely normal thing for silver brazing, but the parts you can actually handle are small. I would not waste money on a cheap OP system or OA, once you want to go there, get a good one, they aren't that expensive, and the performance is night and day. One really doesn't learn all that much about brazing bike parts with propane it is so bad. You might learn you like it enough you are willing to go all in.

Take a #2 pencil and scribe a line as a barrier of where you want the filler not to go. I did this using brass filler on steel and it made a nice fillet.