Squeeze the Base-Coat into the Cracks
Mist the lath with water lightly (Image 1), being careful to avoid over-saturating the wood. This will help to prevent the lath from wicking moisture out of the plaster, causing it to dry too quickly and possibly result in more cracking.
Using a trowel, squeeze base-coat into the cracks (Image 2), making sure to get it into the keys of the lath.
Smooth over the crack and angle off the excess (Image 3).
Depending on humidity, the base coat can take from 2 to 24 hours to set up completely. Allow a couple of days for it to dry completely.
Once the base coat has dried, sand to a smooth finish.