The reason for blunt lines is often lack of proficiency. There is no perfect method. Only by hard training, the wrist must be waved naturally and softly. The rendering of gypsum geometric figures should be the simplest, because they are all regular figures, the rendering must not be too heavy, the wrist should be light, and the transition of light and dark boundary should be presented on the background color (not obvious, but necessary). The opportunistic way is to touch one side with your hand or toilet paper after the tattoo is finished, and the stiff lines will be touched away, which is natural. But this method will not last long, because the problem of stiff lines will not be solved in the middle and late stages of painting, so we still need to practice more. There is no shortcut to any technology. You must practice and draw more, and the quantitative change will produce qualitative change!