I swapped on a Smooth PEI plate I just got from Bambu Store.
I set the plate type correctly in BS.
What do I need to do to make the first layer flat? …aka: avoid the obvious first layer filament tracks?
You can’t unless you want to go trough a lot of trouble and tuning.
Physics is something we can’t always change 
The nozzle has a gap over the plate and the filament fills this gap accordingly.
Means it has a ROUNDED edge.
When the filament hits the previous line it starts to squeeze more to the other side as it has a lower resistance to expand.
There is also the issue of the nozzle being round - the filament can’t come out in other shapes.
A properly calibrated flow ratio helps, same for increasing the first layer extrusion temp but it won’t give perfect sheet of plastic.
The textured plates hide this quite well but on a smooth plate not so much.
If you want them mostly gone you can try this:
Increase the bed temp by some degrees but as high as to soften the plastic to goo.
Increase the first layer extrusion temp by 5 to 10 degrees and reduce the first layer speed by 30 to 50%.
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