On an otherwise smooth and flat skimmed surface I sometimes get a patch of fine rippling. It looks like corduroy has been lightly pressed onto the surface. It catches the light and can be felt by hand. I guess it might be caused by the trowel 'skipping' over the surface but I don't feel this happening nor notice the effect until it's too late to do anything about it. Any idea what it happening and how to prevent it?
It sounds like it could be as a result of your trowel juddering on the wall. This can happen when you are finishing a trowel stroke and almost flicking it off the wall and allowing it to judder. Try stopping dead with firm pressure and avoid the flick.
If you do see any - they can often be sliced off and smoothed out with the trowel even after it has set.