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Synonyms for Undercoat (same or very similar meaning)

WordNet sense 1 (seal consisting of a coating of a tar or rubberlike material on the underside of a motor vehicle to retard corrosion):

WordNet sense 2 (the first or preliminary coat of paint or size applied to a surface):
ground, primer, priming, primer, flat coat, primer coat, priming coat

WordNet sense 3 (thick soft fur lying beneath the longer and coarser guard hair):

WordNet sense 4 (cover with a primer; apply a primer to):

From the ODE community, based on WordNetadd/edit