When over-printed with a black ink, we often have pinholes. How can we eliminate the pinholes?

See the list below for causes of pinholes:

1. The first layer is too thin (less than 5 µ)
2. Too much dilution of the ink (more than 20%)
3. Too much squeegee pressure
4. Under layer is pale colored, such as a medium based color
5. Any combination of these causes