Is it worth undercoating an old car?

Whether it is worth undercoating an old car depends on condition of car, how long you plan to keep it, and climate. If old car is already heavily rusted with holes in floor or frame, undercoating is not worth it and can even make things worse by trapping moisture under coating and accelerating hidden rust, so money better spent on repairs or replacement. If old car has light surface rust but is structurally sound and you plan to keep it 2 to 5 more years, especially if you live in rust belt with salty winter roads, undercoating can be worth it to slow further rust and extend life, costing 150 to 400 dollars versus buying another car. For old car that is classic or sentimental and you want to preserve, undercoating plus rust treatment is worth it. Oil based undercoating that creeps into seams is better for old cars than hard rubberized coating which can trap moisture over existing rust. You should first remove loose rust, treat with rust converter, then undercoat. If old car is near end of life and you plan to replace within year, not worth undercoating. So for old car in decent condition that you want to keep few more years and live where roads are salted, yes it can be worth it, but for heavily rusted car ready to retire, not worth it.