A garden sprayer awaiting repair emerged from the benchtop clutter. It’s an old one, with a metal shell and actual screws, so I could dismantle it to reveal the problem:
It’s evidently impossible to make a good, cheap, corrosion-resistant spring (pick any two, I suppose):
Some rummaging in the Big Box o’ Medium Springs produced a slightly smaller spring that should last for a while; it’s good, free, and rust-able, if a bit too short.
Much to my astonishment, I found a length of 3/8 inch Marine Bronze rod in the stockpile and made a bushing to take up the remainder of the space:
It won’t get a good test until gardening season opens next year, but it seems to seal well enough.