The story
Said to be from French beau montage, "beautiful face", referring to the appearance of the filled surface. https://books.google.co.uk/books?id=6vA7AwAAQBAJ&pg=PT157&lpg=PT157&dq=%22beau+montage%22
Etymology from Wiktionary, CC BY-SA 4.0.
beaumontague
Today it means: A cement used in making joints, filling cracks, etc. For iron, the principal constituents are iron borings and sal ammoniac; for wood, they are white lead or litharge, whiting, and linseed oil. Full definition →