Proper methodology is to wipe off both the chip and the sink, reapply the thermal compound. If a super thin layer remains behind, more like a stain, it's OK, just reapply as usual......
The biggest disadvantage to as3 is that it's hell to clean off completely. If you've used it once, you already know that, though....
I'm sure that thugsrook's method will work as well, if the original compound is still creamy and of the same consistency as new stuff....