How do I repair a small nick in motor windings?
As long as the conductor in the motor winding is undamaged, you can probably repair the insulating varnish with a product that is made for that purpose.

Here is the tech sheet to one such product, MG Red Insulating Varnish 4228:

http://www.mgchemicals.com/downloads/tds/tds-4228-l.pdf

Amazon stocks and sells 55ml bottles of this for about $10.