Maybe this does not apply to your situation, but In CURA I’ve been running everything using Perimeters before infill.
I see from time to time depending on print and material, that I get Zits sometimes on the outside of some of my prints.
I think what is happening and have yet to really confirm it, is that at certain times, when infill stops to move over, I think that
the extruder is turning off the filament feed, but it takes a short time to stop and change direction.
If, in your case infill is running before perimeters, it could possibly be that the filament is turned off and the extruder travels a bit, which
I think could be leaving strings.
I do not have a Wanhao printer, so I may be very wrong in your situation, But that’s my two cents worth.
I hope it helps you in some manner.