I've never tried this with Helli Canopies but I use the method constantly for other small models that I paint. Oven-Off is about the most caustic cleaner you can legally buy, just test on a small piece of canopy material to make sure it's compatible with the plastic. It's extremely caustic and you should use precautions when handling (Mask and gloves designed for caustics as well as safety glasses) But cleans about as good as a cleaner possibly can, we use slightly modified versions of this stuff at my work to clean just about anything off steel parts. I do stress to be careful with it though, it dissolves fat/oils in skin causing chemical burns pretty easily.