A modern car will have sufficient rustproofing. If you want to do an older car it needs to be waxoyled & injected into box sections. As pointed out painted on black underseal traps water underneath & panel will rot out underneath.
I have mine done every 3 years by a guy in Ashbourne, he has it for about a week when he does it.
Steamed off, dryed, and then waxed inside chassis and out.