I had that sticky, dripping tar like stuff not that long after I got my WSM. I don't remember how many cooks I did up to that point, but that stuff just dripped off the edge of the lid and was nasty. I did a burn out with the lid on top of the charcoal bowl with plenty of hot coals and let it go. A day later, I scraped the whole inside of the lid with a putty knife. What I had was a black layer, followed by a brown layer, then the real inside of the lid. I got the outer layer off, then the brown layer and it wasn't that much work, maybe an hour of scraping then rinsed. I don't know what caused this, but others a few years ago reported the same thing, but only a few. I've been fine since. FWIW, I never fully close the bottom vents and leave the top vent fully open all the time. I don't do anything differently now than I did then so who knows why this happens. Now, about once a year, I do a scrape job on the lid.