Cause
Two problems in one. General poor application causing the material to sag and shrivel. Rivelling is caused by the film of a thick coating drying too quickly and skinning before the underlying material is dry.
Remedy
Once the entire coating is dry and hard, flat down using wet or dry carborundum paper, wet, with warm water and add soap. Thoroughly rinse with clean water and allow to dry before re-painting with additional coats.