Ok...I have used this technique only a few times with varying degrees of success. First, lightly and only in a few key spots, PVA some thread in the panel groove. Once that has tacked up next putty the entire gap and smooth as best you can. After that, but before the putty has set-up, carefully and lightly pull your thread out. You'll be left with a very thin gap where the thread came out which you can deepen or widen. Practice a few times!