So it was the start of summer and a mates tractor tyres were getting a low for road use (visually at least), but he didn't want to fit new tyres until the start of winter - "did I know anything about regrooving?"
As it happened, I'd seen it done not long ago and it looked easy enough (so long as you don't go too deep) and went searching for someone selling the tool.
Surprisingly expensive, was the answer.
"Can you think of another way?" quickly followed - (apparently I could).
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If you ever do this, do it outside someone else's house.
The 'swarf' comes off hot and vulcanises itself to everything it touches.