A dull chainsaw is not only ineffective, but dangerous. You can sharpen it by hand or use specially built tools to save time and mistakes.
A chainsaw can dull from lack of lubrication, applying excess pressure, and from hitting dirt or rocks. Always check that your bar oil is level and use proper technique when cutting.
You can sharpen a dull chainsaw blade using a variety of hand files and attachments that serve as angle guides. This process is time consuming, but inexpensive. You can grab a chainsaw sharpening kit for under $20. Unless you are experienced, filing by hand can do more damage than good, though, so take your time.
In this video, Rick Pickford walks us through the intricacies of hand sharpening a chainsaw:
Alternatively, you can use a grinder that is specially built for the task. The Grandberg Grind-N-Joint is a 12-volt chain grinder that can run off your truck battery, so you can sharpen in the field. This video demonstrates how to set it up and use it: