Masking canopies with a liquid masking compound is difficult because it tears and cuts easily. You can find Ambroid liquid mask in Hobby Lobby. I recommend Micro Mask which can be found in hobby shops and online at stores like www.greatmodels.com and www.squadron.com.
Today there is a new way of masking canopies and that is to buy a special die cut mask made for a certain kit. Just apply the masks and paint. Eduard and Cutting Edge Modelworks both produce masks. www.greatmodels.com sells masks from both lines and is a great company.