One that hardens, especially a substance added to varnish or paint to give it a harder surface or finish.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "hardener" in a sentence
"Bondo is a sort of goop, mixed from polyester resin and a hardener, that is used to fill in dents and dings in battered race cars."
"He acknowledged that there are times when he uses a performance-enhancing substance: nail hardener."
"It comes in two tubes, one with steel dust and then a hardener."