What will improve this ?

[Bikeracer2020]

I've started using Prusa Slicer and it is a slicer setting, so nothing to do with boolean operations or polygons in the STL mesh, latest test print has removed all the ridges in the top surface finish by using a Monotonic fill setting.

The cause of the ridges was because the extruder moved left to right, stopped and printed in a different area and then resumed by coming back going right to left until it reached where it had stopped previously.

The Monotonic setting makes the extruder always move in the same direction, so it always restarts extruding where it left off.