Yet another rust question (3M Undercoating)

[attworth]

I plan to use 3M undercoating to coat the underside and engine compartment, and probably any other unpainted surface I can find on my car. I was wondering if this stuff actually "stops" rust from getting worse.

 

I.e., say there's a bit of rust on the inside floor of the car. Not through the surface rust, still solid metal. Is it okay to spray this directly on top of that? Or would using POR-15 or something similar be a better idea in these areas?

 

Here's what I pulled from 3M's website.

 

"VOC compliant product. Black, multi-purpose, low cost, aerosol undercoating. Specifically used for undercarriages, wheel wells, frame rails, and fenders. Designed to protect metal from corrosion and abrasion. Can also be used as a sound dampening coating."

 

Does not really mention "killing" rust.

 

Edit: Is it okay to spray this on top of the old undercoating? As in, if I can tell without a doubt that there won't be any rust under the old coating, is it ok to spray this on top of it, as well as on top of the bare metal?

[ckolander]

I don't actually have any experience with this, but IMO it would be a good idea to get rid of the rust and old undercoating before you spray on the new stuff. I believe the reason that rust is called cancer in the automotive world is because once you have it, it gets worse until you cut it out and actually fix it. If it's just light surface rust, take some sandpaper to it and get the rustoleum rust 2 primer spray. That stuff worked really well for me.

 

Chuck

[palosfv3]

Any of the 3M undercoating or chip coating (3m-8964) products have no rust inhibiting properties . They seal out most moisture but if you apply it over rusty metal it can, in certain exposures, actually trap moisture and accelerate the rusting process. If your really serious about things . Clean all rusted surfaces to bare metal by sandblasting or other mechanical means , treat with the proper metal conditioners and apply a zinc rich epoxy primer ,sealed with some type of top coat before applying any undercoating or chip guard. I prefer the chip guard since it hardens and is paintable later . The undercoating just doesnt perform as well .

[AK-Z]

I had this stuff on my car prior to me owning it. When this stuff scratches and "heals" it self over, it gives moisture opportunity to become incased and will eventually reach the paint. Since you are in texas I don't think its a big issue, Its just a hassle and a mess to put on.

[palosfv3]

I had this stuff on my car prior to me owning it. When this stuff scratches and "heals" it self over, it gives moisture opportunity to become incased and will eventually reach the paint. Since you are in texas I don't think its a big issue, Its just a hassle and a mess to put on.

 

Many areas of the US are "dry climate " and some areas have minimal rust issues due to their location. Any time you can see condensation on the exterior of your automobile you will find the moisture actively accelerating the rusting progress regardless of locale. If you can see condensation on the exterior of the car you are pretty much guaranteed that the inner side of the same or adjacent panel is wet. It is this inner panel moisture that is the primary cuase of panel rust through since the moisture is trapped in the corners and crevices of the inner areas of the car and these inner areas dont have the air flow necessary for the moisture to evaporate quickly. Cars from the rust belt states or coastal areas exhibit this type of damage quicker because of the salt in the enviroment and the higher exposure to moisture. Being in Texas doesnt make a car immune from rust. It just has a lower exposure to the conditions that cause it . Unless its near the Gulf .

[attworth]

Thanks for all the input. I still have a TON of Por-15 leftover un-opened. I think I'll use that instead after stripping the undercoating.

 

Texas does has rusty cars, just not nearly as bad as other areas. Many people think its okay to store their cars in a wet pasture on flat tires. Nothing like tall, wet grass rubbing the bottom of your car for a few years.

[AK-Z]

Many areas of the US are "dry climate " and some areas have minimal rust issues due to their location. Any time you can see condensation on the exterior of your automobile you will find the moisture actively accelerating the rusting progress regardless of locale. If you can see condensation on the exterior of the car you are pretty much guaranteed that the inner side of the same or adjacent panel is wet. It is this inner panel moisture that is the primary cuase of panel rust through since the moisture is trapped in the corners and crevices of the inner areas of the car and these inner areas dont have the air flow necessary for the moisture to evaporate quickly. Cars from the rust belt states or coastal areas exhibit this type of damage quicker because of the salt in the enviroment and the higher exposure to moisture. Being in Texas doesnt make a car immune from rust. It just has a lower exposure to the conditions that cause it . Unless its near the Gulf .

not necessarily true. My car was only rusted on the lower 3 or 4 inches. This is do to SNOW and ICE scratching away at the undercoating and then re-incapsulating the moisture (and salt). Under coating is primality used for sound and heat insulation, as well as scratch prevention. Not for waterproofing. If you really want, you can undercoat it with some bed liner stuff. I just saw that they sell it in a spray can now. $11 a can where I saw it.