Phosphate and oil is cheap but as noted it will not last.
In the past I soaked the bits in citric acid for a day or so to remove the rust, then soak in phosphoric acid until black. Heat the oven as high as you can get it and cook the bits in sump oil. Remove and drain the bits.
I had my nuts and bolts treated and cadmium coated by a guy at Salisbury.
Cost me about $40 for a bucket full if IRC.
PM me if u want the details.
Only down side was all that all the previous labelling of bolts went out the window and I had to remember what they came off for reassembling.
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