What causes this surface finish?

The extruder is a direct drive, 200 steps/rev at 16x microstepping running on an Azteeg with Suresteppr drivers. I was thinking that maybe it is due to the resolution of the extruder motor, that there are few enough steps to see some pulsing in extrusion. However other direct drive printers like Replicator don't seem to exhibit this. So maybe there is some interaction with XY movement. Any ideas how to smooth this out?

That might be the infill pattern showing through the walls. This can happen if you're extruder is not calibrated properly and your're extruding too much plastic. How does a single-wall object look?

Chasing that in my prints as well. What size belts are you running?

Got some advice last week to switch to GT2 belts and gears from the T5 ones I am running now. I can see the belt 'humping' over each tooth on the gear and idle pulleys during printing.

Can't confirm yet, new belts on order.