I do what you did. I wash the garment with the interfacing on it. Sometimes it just peels off, and sometimes it doesn't. On the stuff that does not easily peel off, I use a steam iron with lots of steam on the interface side. Be very careful of the heat, but you need to try and remove it while it's hot. If that doesn't work, I just use the portions of the garment that were not interfaced and discard the rest.