craftgrrl | Fusible interfacing on a knitted item?

I have a quick question for everyone here.

I bought my boyfriend a sweatervest, and decided to put a snowflake on it (I assure you it is more awesome than it sounds). I had seen a program about faking intarsia by replicating the knit stitches in a new color, which is what I'm doing -- I'm sure this has a name, but I have since forgotten it. Unfortunately, the wrong side of my creation is looking a little...scary. Somewhere in the center, the ends of the embroidery floss got weaved in and stuck in loops and I had to cut some things to make it not look like a disgusting hairy knot. But this means smaller ends that I fear might unravel through the knitted sweater. I have some fusible interfacing lying around, and was wonder if me ironing it to only the sketchy center of the inside of the snowflake, thus capturing all the ends and locking them in place, would work to correct for my mistake. If not, what can I do? Also, if the interfacing will work, can the item be machine washed? (I am going to ask him to hand wash it, but you know.)

Thanks for any input, and I promise to post pictures of the finished item. :)