(as) close as two coats of paint
Having a very close relationship; inseparable. I haven't talked to Patty in years, but we were as close as two coats of paint as kids.
*close as two coats of paint
Cliché close and intimate. (*Also: as ~.) When Tom and Mary were in high school, they were as close as two coats of paint. All their lives, the cousins were close as two coats of paint.