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Either way it’s a pain in the arse.
Mark it up first with a marking gauge or mortise gauge.
Rent a router and use a bit narrower than the groove width you’re aiming for.
You’re still going to have to finish the last 6” with hand tools.
If you try to create a 10mm groove with a 10mm bit it will not work, you have to use a narrower bit and relieve each side.
You cannot use a router bit in a drill, they are all too slow and uncontrollable.
Or... a chisel and mallet.This is a rebate.
https://www.wonkeedonkeetrend.co.uk/media/wysiwyg/Trend-router-cutters/rebates.jpg
I need to slightly widen a 10'ish mm rebate in a door frame (it holds an intumescent fire strip).
Ideally, a 10mm straight router bit would get rid of the paint and widen it enough but I don't have a router.
Apart from sanding down the entire rebate (which would a PITA and take ages) is there a crafty way to do this?
I've considered getting a router bit, putting it in a drill and letting the existing rebate guide it.