What is the reason for the surface of the aluminum profile at normal temperature blackening? How to deal with it?

Tel motiv:
1 grăsimea, rugina și scala nu sunt îndepărtate;
2 After the blackening, the residual liquid is not washed;
3 parts are shaking too fast;
4 parts overlap or fail to flip.

1 strengthen pre-treatment, degreasing and pickling;
După întărire, lichidul rezidual trebuie spălat;
3 parts shaking speed should be appropriate;
4 parts should be flipped to prevent overlap.