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rotorwrench · 04-29-2021, 10:13 AM

Undercoatings were advertised to reduce road noise and some may have advertised corrosion protection but that would be false advertising in most cases.

I have two 1951 Mercury cars. Both came from the St Louis, plant in December of 1950. One was undercoated and one was not. The one that was undercoated rusted out in both inner and outer rocker panels with a lot of floor rust as well. The one with no undercoat rusted in only a few places on the inner and outer rockers with virtually no major rust on the floor. What happens is the undercoat hardens and starts to get fractures in the coating over a period of around 5 to 8 years and it starts holding moisture between the coating and the steel. If a car is driven in winter where salt is used then the salt brine starts getting in there too. This can literally eat the steel up and leave nothing but an undercoat shell with iron oxide in there.

If the car is garaged and never driven in inclement weather then it may still last a very long time as long as all the steel that it is coated has good primer and paint adhesion at the time of the coating deposit.

04-29-2021, 10:13 AM	#7
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,424	Re: Undercoating would like your comments. Undercoatings were advertised to reduce road noise and some may have advertised corrosion protection but that would be false advertising in most cases. I have two 1951 Mercury cars. Both came from the St Louis, plant in December of 1950. One was undercoated and one was not. The one that was undercoated rusted out in both inner and outer rocker panels with a lot of floor rust as well. The one with no undercoat rusted in only a few places on the inner and outer rockers with virtually no major rust on the floor. What happens is the undercoat hardens and starts to get fractures in the coating over a period of around 5 to 8 years and it starts holding moisture between the coating and the steel. If a car is driven in winter where salt is used then the salt brine starts getting in there too. This can literally eat the steel up and leave nothing but an undercoat shell with iron oxide in there. If the car is garaged and never driven in inclement weather then it may still last a very long time as long as all the steel that it is coated has good primer and paint adhesion at the time of the coating deposit.