You should mean slag inclusion. After slag removal, the weld is not smooth enough and intermittent in the middle. My buddy tells you how to solve it. Go back and try. Turn up the current, the bigger the better, as long as you don't burn the iron plate through. For example, an iron plate with a thickness of 8 mm, if it is AC welding machine covered electrode 3.2, the current is 160~200, if it is DC welding machine covered electrode 3.2, the current is above 150. Arc pressure must be very short, to what extent, it is no problem to put covered electrode directly on the iron plate, long arc is a no-no, and one more thing, welding speed must be slow, too fast is also a no-no, in fact, in addition to flat welding, fillet welding is the best welding. Anyone can burn beautifully as long as the current is adjusted sufficiently. Principle: The greater the current, the more heat covered electrode releases per unit time. With proper angle and speed, the iron plate can be easily melted without being disturbed by slag. As long as the iron plate can be melted and solidified, a continuous weld will be formed, and the problem of "holes" will not occur.