To clean up pedal threads, you start with a couple of steel crank arms, left and right, of course.

Start the pedal spindle, in the appropriate crank arm, and thread in, by hand, until it stops. Holding the crank securely, with your hand, smack the crank arm, on the outside diameter, of where the pedal threads in. Don't be shy. Do this in several spots. Now try the pedal again. It should thread in a little more. Needless to say, the crank arm(s) will be somewhat banged up by the time you finish.

Keep doing this until the threads are free and that should do the trick. And, speaking of tricks, this is an older Millwright trick I learned long before I became an old Millwright.

Make sure you install the pedals with greased threads, when ready for installation on the bike.

When cleaning up an alloy crank thread, use an unwanted pedal spindle, left or right hand as required, and slot the threads, as suggested above. The threads will clean up nicely. This time use a lubricant, while chasing the threads.