Swedish to English Tech/Engineering Materials (Plastics, Ceramics, etc.) Items requiring extra care when painting

På spikskallar och kvistar kan färgen smita vid första målningen, men täcks vid ommålning.

This is a very disjointed text, basically instructions for applying paint and other forms of wood treatment. It is intended for DIY readers at least as much as professionals.

Are spikskallor nail heads?
There are several references to ´dutta färg på spikskallar o. dyl. ´
I can imagine that the paint etc. does not cover properly at first, but with an extra thick layer, it fills in the dent or sets over the bulge anyway.

I´m sure most handymen/women just know that sort of thing, but thanks for any help!