This remark refers both to the success of the wet-on-wet technique practised in the Netherlands at that time, as well as the fact that the trekschuit, which was a new invention in 1632, allowed regular comfortable transport between Haarlem and Amsterdam and made such trips to portrait painters possible.
This remark connects the success of the wet-on-wet technique with the accessibility of travel made possible by the trekschuit, both contributing to the ease of transporting artists for portrait painting.