I was asked this recently by one of the new fellows at my work when I pointed out he needed to make sure all the welds got chipped. (That slag covering on the weld is the flux coating on the welding rod after it melts, combines with dirt & impurities on the pieces being joined, floats to the surface & cools. It's an essential part of the stick and fluxcore welding processes. MIG welding is a different process & doesn't produce this slag covering on the finished weld) I could have just said "Because you have to!", but that isn't much of an answer. There are several reasons: If the job is being done to CWB (Canadian Welding Bureau) specs, an example being structural steel fabrication, all welds must receive at least a visual inspection. You can't inspect what you can't see! An engineer or steel inspector can cite unchipped welds as a deficiency - I've seen it happen! If the weld you are doing requires multiple passes, the slag must be chipped away before laying another pass down. Remember that dirt & impurities are contained in that slag, if you weld right over top they will be trapped in the weld. A large number of welder test plates that fail the X-ray examination do so because of "slag inclusions", tiny bits of slag that didn't get cleaned off & were trapped in the next layer of weld metal. And, it looks a lot more professional. Rough-looking unchipped welds are NOT something I want to leave as a calling card.