The drums don't need to be baby butt smooth to work with new shoes. The grooves can be shallow and they will take a little longer to break in. I will usually roughen the surface to aid in break in. The ones that really need turning are the ones with deep grooves and ones that were run with the lining completely gone. Also any wedge shaped wear or curved shaped wear would be a reason to turn a drum. I would worry more about the magnet face not being smooth. If you put new magnets on an old drum, any out of flatness will really mess things up. It is best to replace the magnets and drums as a set. Yeah you can grind the magnet face but how much is too much metal gone. I would expect that braking force is a function of how far that magnet has to move to stick to that drum face. At low voltages it may not stick at all because it is too far away.
None of my drums had large grooves but they seem to be taking their time breaking in. Two trips down and still not 100%. I hope they get it together before my cross country jaunt in a couple weeks.
If you can find it,(still got mine), coat the shoe or pad facing with a light coating of valve grinding compound.
Five stops after my Dexter shoe replacement and we were normal.....well the brakes anyway.
Bob
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There is a way to contour the shoes to fit the drum. This can be done when fitting new brake shoes.
Put the shoe in the drum and see how it fits. Probably it only touches in the middle. Sand down the area where it touches, with a sanding block or belt sander. Keep checking until the shoe makes complete contact.
Brake shops used to have a machine to do this.
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