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The first type is biting the bottom of the substrate. Especially for some plastic substrates, which are quite common, such as PS, PC substrates, etc., as well as some foamed plastic equipment, they are easily bitten. The main reason is that our solvents have strong solubility, especially for some esters, aromatics, and ketones.

We can change the solvent system, such as using alcohols like isopropanol and isobutanol. Or alcohol ether solvents, such as PM, BCS, etc., can be added with fatty hydrocarbon solvents, which are non bottom biting, such as white oil, D40, D60, etc. It can effectively solve the problem of bottom biting. Ketone ester solvents such as IBIB isobutyl isobutyrate, MEK, DIBK, etc., which are not easy to bottom biting, can also significantly reduce the phenomenon of bottom biting. In addition, we can also use fast drying esters or aromatic solvents to evaporate before the solvent can shake to the bottom. It can also effectively avoid the problem of biting the bottom. For example, we can use volatile solvents such as ethyl acetate and toluene in our formula.

The second type of undercutting is when the upper coating undercuts the lower coating. The coatings on the upper and lower layers may be of the same type or different types. The coating shows the surface of the paint film, the raised or wrinkled paint layer below, which may be broken to the outermost layer. What are the main reasons for this? There are several types.

The first step is to apply the second layer of paint before the paint film has dried. Primer, completely dry is actually no problem. When wet touched or sprayed, there is no problem. When the first layer of paint film is still wet, the upper layer of paint is applied, and the solvent of the upper layer of paint will melt and expand, damaging the air film that has not fully cured in the lower layer, causing the paint layers to bite each other, resulting in a phenomenon of biting the bottom.

The second issue is that if the primer and topcoat are not compatible, this phenomenon of biting the bottom may also occur. For example, after drying, the primer cannot withstand the erosion of solvents contained in the topcoat, making it particularly prone to biting the bottom. Another fruit is the topcoat, which is prone to shaking when using solvents that are too corrosive or soluble to the primer.

The third issue is that the paint film is too thick. Insufficient leveling time can also cause a decrease in the adhesion between paint layers, while the primer may dry out, which is likely to result in poor quality and cause foot problems.

The fourth issue is the use of inappropriate diluents, such as diluents for primer that evaporate too slowly, diluents for topcoat that evaporate too quickly, and solvents for primer that cannot evaporate in a timely manner, resulting in separation between the internal and surface paint layers.

Thus forming a bite on the bottom. The slow evaporation of the diluent for the topcoat can also cause solvents, which can prolong the erosion time of the lower layer and cause bottom biting. The fifth reason is that the topcoat dries too quickly or the temperature in the painting workshop is too high. The newly sprayed paint layer dries quickly in a short period of time, causing the surface paint to shrink quickly and open faster than the primer. As a result, the topcoat has dried, but the primer has not yet dried. In the subsequent drying process, due to its varying degree of shrinkage, it can also cause a phenomenon similar to biting the bottom.

The sixth is that if the primer is too soft and the topcoat is too hard, the final shrinkage of the topcoat will still be different, which can also cause a similar phenomenon of biting the bottom. This principle can also be used to prepare water-based crack paint.