Generally speaking, the traditional bag filter and melt blown filter can intercept particles with a high proportion size smaller than the filter element precision. In the coating production process, this kind of clarification filtration is not expected to happen, because it will waste a lot of effective ingredients such as pigments, metal powders, mica and fillers that are expected to be retained. The result of using this filter is poor coating effect and short service life of filter element. The staged filtration can avoid the removal of effective components and extend the replacement cycle of filter element properly.

The selection of bag material is the first consideration for bag filter. The success or failure of bag material selection will directly affect the filtering effect. The wrong material of filter bag will lead to material pollution and decrease of filtering effect. The main consideration of material selection is the compatibility of filter bag and operating medium. Generally, through the characteristics of the operating medium, design temperature, design pressure, pH and operating conditions (such as whether it is necessary to withstand steam, hot water or bactericidal chemicals, etc.), evaluate one by one, and remove the unsuitable filter materials, i.e. initially select the available filter materials.

The use is also an important consideration, for example, the filter material for drugs, food or cosmetics must be FDA approved; the ultra pure water must be selected as the filter material that is pure and does not contain the release material that will affect the specific impedance; the filter gas must be selected as the hydrophobic material and whether the "sanitary grade filter" design is required, etc. According to the size of retained particles proposed by the process and the efficiency curve of the filter bag provided by the manufacturer, appropriate filter bag aperture shall be selected. Again, we have to mention two concepts of filtration, namely, filtration and relative filtration.