This is my ally.
I can vapour blast nuts and bolts but it’s a bit of a prick and my hands cramp up from holding them under the blast stream.
But they do come out very very clean.
Soda. It’s a slow processes and is very messy unless you have a cabinet.
Cheap place to buy from are pool supply companies. My day job is in a council pool and we use bicarb a lot for cleaning and pool alkalinity. Different suppliers have different sizes. but we buy it by the 500kg pallet so my price is very different. Its feed grade not food grade so that part of the cost is not there. As you are not eating it no point in paying for it.
I buy it in 25kg bags. Im happy to help out and supply small quantities for a good price if you want a kg or two. PM me.
Im currently looking in to a tumbler. Enabling me to clean up bolts and small parts to go to platers after. All steel parts get washed in a rust blocker that is coating safe so gives a bit of time before rust starts again. Looking at videos on you tube you would have to leave the bolts in the tumbler for many many hours, days even, in order for any damage to occur on the threads. It’s a slow process.
Plating in Auckland
I have heard and seen good results from Terei brothers. They still take small job lots from car guys, many others do not.