WordNet records no synonym and no antonym for buildup. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the act of building up an accumulation
“I envied his rapid buildup of assets”
the result of the process of accumulation
“the buildup of leaves blocked the drain pipes”
Synonyms, antonyms and the broader and narrower relations for buildup come from Princeton University’s WordNet. Full sources and licences.
highly favorable publicity and praise
“his letter of recommendation gave her a terrific buildup”