4. Common problems for coating materials
It is concluded that the common problems for coating materials can be roughly divided into three types, which are uneven coatings, dripping of preservative and foaming of preservative.

Uneven coatings
preservative is unevenly distributed on the surface of the steel pipe, resulting in too thick coatings on some parts, too thin coatings on some parts and no coating on some parts. As a result, thickness of the too thick coating exceeds the standard, which causes waste. And the areas that are coated too thinly or not coated will cause reduction of the anti-corrosion ability of the steel pipe, eventually causing rust.

Preservative dripping
The preservative is solidified on the surface of the steel pipe like water dripping, which is preservative dripping. The occurrence of this phenomenon often does not directly affect the corrosion resistance and can also ensure the corrosion resistance required by the steel pipe. However, from an aesthetic point of view, the steel pipe with preservative dripping looks matt and uneven, which directly affects the appearance of the steel pipe.

Preservative foaming 
Air enters the preservative agent, so foams are formed in the coating of the steel pipe. These foams are different in size according to the specifications of the steel pipe. The shape of the big foam is like the bubble on the protective cover of some home appliance remote controllers. The foam will be broken by pressing by hands. The preservative foaming affects the appearance of the steel pipe, making the entire surface of the steel pipe rough, and the breaking of foams will lower the standard of the coating film thickness, reducing the anti-corrosion ability and making corrosion on the foam part of steel pipes.
 
5. Cause analysis of quality problems
The quality problems that emerge from a bundle of steel pipes may also be a combination of different causes. The causes of uneven coatings can be roughly divided into two types. One is unevenness caused by spraying after the steel pipe enters the coating box, the other is unevenness not caused by spraying. When the steel pipe is at the coating box, there are 6 spraying guns around the coating box. If the coating flow of each spraying gun is different, it will cause the uneven distribution of the preservative on each surface of the steel pipe. There are many reasons for uneven coatings not caused by spraying. The incoming steel pipe rusts and rough, which makes it difficult to evenly distribute the coating. The left emulsion is on the surface of the steel pipe when the water pressure is measured. At this time, the coating is in contact with the emulsion, so that it is difficult for preservative to adhere to the surface of the steel pipe, resulting in no coatings for the steel pipe part with the emulsion and uneven coatings of the entire steel pipe.
 
Reasons for dripping of preservatives
The cross-section of the steel pipe is circular. Each time when the preservative is sprayed on the surface of the steel pipe, the preservative on the upper and edges will flow to the lower part due to the force of gravity, thus forming dripping. Fortunately, there is oven equipment for the coating production line of the steel pipe factory, which can heat and solidify the preservative sprayed on the surface of the steel pipe in time to reduce the fluidity of the preservative. However, if the viscosity of the preservative is not high, the preservative is not heated in time, the heating temperature is not high or the nozzle is not in good working conditions after it is coated, the preservative will drip on the steel pipe.
 
Causes of preservative foaming
Preservative foaming is caused by air humidity of the environment of the work site, the coating being over-dispersed and the temperature drop during the dispersion process.