You won't over-wax--excess wax will shoot out quick. Don't use excessive heat--you can damage the fibers in the string. If you use a piece of leather to burnish the string, be careful. Bare fingers are the safe bet--you'll quit before it gets too hot. Softer/synthetic waxes penetrate better than pure bee's wax. If you think your string needs waxing, it probably does.

Chad