1. Weld defects: The weld defects are serious, and manual mechanical grinding is used to compensate for them. The grinding marks produced cause uneven surfaces and affect the appearance. Pickling and passivation of the weld alone also cause uneven surfaces.

2. Scratches are difficult to remove: Overall pickling and passivation cannot remove various scratches produced during the processing, and cannot remove impurities such as carbon steel and splashes that adhere to the surface of the plastic-coated steel pipe due to scratches and welding splashes, resulting in chemical corrosion or electrochemical corrosion and rust in the presence of corrosive media.

3. Uneven polishing and passivation: Pickling and passivation treatment is performed after manual polishing. For workpieces with larger areas, it is difficult to achieve uniform treatment effects and an ideal uniform surface cannot be obtained. In addition, the labor cost and auxiliary material costs are also high.

4. Limited pickling capacity: Pickling and passivation paste is not omnipotent. It is difficult to remove the black oxide scale produced by plasma cutting and flame cutting.

5. Scratches caused by human factors are relatively serious: During the process of hoisting, transportation, and structural processing, scratches caused by human factors such as bumps, dragging, and hammering are relatively serious, making surface treatment more difficult and also the main cause of rust after treatment.

6. Equipment factors: During the bending and folding of profiles and plates, scratches and creases are also the main cause of rust after treatment.

7. Other factors: During the procurement and storage of plastic-coated steel pipe raw materials, the bumps and scratches caused by hoisting and transportation are also relatively serious, which is also one of the causes of rust.