It depends how much you use a tool and what you are digging into. For what I do, I maybe get 2 years worth out of the paint job before reapplying. You gotta realize that planting in cemeteries is really taxing on tools. Big time. And it's for the simple reason that backfill over graves and under gravestones is usually gravel which tears the crap out of any paint job. Also, the "top soil" is usually not so much soil as it is clay, sand and whatever else is leftover from opening the ground up. It sometimes takes years to get the soil to a proper consistency by incorporating peat moss and other amendments to loosen it up. Of course, once it's loosened up you'll get lots of weeds because they'll get an easier foothold. Tradeoff, the saving grace is that it's easier to remove the weeds too.
If you use a decent paint, it shouldn't come off much if used in sand occasionally. All the time? Well, then I probably would opt not to paint as the sand will clean the tool as you use it.
Again, I'm normally using the tools in the cemetery where it doesn't matter if a chip of paint flakes off. The amount of chemicals coming off those small chips is probably less than the chemicals found in the soil where you are planting your salad greens. If you worry about it, then there are other methods.
- Don't paint the tools, let them be and just sand off any rust at the end of the year.
- Use plastic tools, though I'd worry more about the chemicals in the plastic than metal. And breakage.
- Use aluminum tools. Might be more expensive and probably less sturdy than it's steel counterparts and they will wear quicker.