Using a piece of leather can damage the string. It's basically nylon and too much heat will melt it along with the wax. If you just use your fingers you can tell when it is hot enough. Actually putting too much wax on will slow your cast because the string needs to flex as it moves forward. If you think you may have too much wax, just get a length of dacron or strong cotton thread and knot it around the string. Then drag it up and down the string a few times. This will scrape all the excess wax off and clean the look of the string in the process.

I use B-50 & wax it once a month or before hunting in wet ( fog/misty rain/snow )weather.

Just did - it's been a while.  Thanks for the reminder.  That string was looking skeezy.  :-D