Primer under Undercoating

[Guest thinkmonkey]

My car is stripped to bare metal and I am going to be priming it. Will the undercaoting stick alright on top of the primer or should I mask off the bottom/undercoat it right now? Thanks guys.

 

-David

[blueovalz]

I don't understand the cause, but I have witnessed the effect of well prepped bare metal being undercoated, and then years later having it almost fall off because of rust that developed under the undercoating in wide spread areas. Someone know why??

[Guest Anonymous]

Terry,

 

I have a theory about the rust growing under the undercoat.

 

Think of the undercoat as a vapor barrier. Once moisture finds it's way between the metal and the undercoat, the moisture spawns rust. I think of rust as a "living" entity. Once it has moisture and food (metal),it will grow. As the rust begins to grow, it will force the coating away from the metal because it wants that steel food to grow bigger. It nibbles away at the edge of the undercoat, like picking away a scab. Pretty gross.

 

This conversation is also taking place about Mikelly's Z, only in his case he has rust growing underneath bedliner paint. See the thread:

http://www.hybridz.org/ubb/ultimatebb.php?ubb=get_topic;f=9;t=001048

 

I believe the bottom line is, that you can either slow rust down with chemicals, paints, or stainless. The Delorean was a step in the right direction. Well, a step anyway.

[Guest Anonymous]

Hey racerX,

Are there different kinds of undercoating? What kind of undercoating do you recommend and can I buy it over the counter at a local auto paint store (I have seen some in the spray cans)?

 

Whats is the best way to apply the undercoating, who many undercoating layer coats can you applly (is more better)?

 

Thanks Fletch

[Guest 240hybrid]

I would recommend POR-15 rust preventive paint. It doesnt leave pores like reg. primers and paints do which allow moiture and air to penetrate them. They also claim that their product will not chip, peel, or crack. I would use this and then use a undercoating over it. POR-15 also sells prep supplies which will also help extend the life of whatever you use, and weather you use a undercoating or not. They say you can paint over rust and have no more problems, but I wouldnt bet my Z on it. I would remove any rust visible before coating, which their "METAL READY" product is good for, which removes light surface rust and also preps for paint. Just as everyone else says though, undercoating isnt a rust preventor, just a noise killer. Good luck

[Guest Anonymous]

It is a common misconception that if you put lots of paint on, it will stop rust. This is NOT the case!

Even modern 2-pack paints allow the ingress of water through to the metal. Assuming the bare metal is totally rust free (if it has been stripped of paint for any length of time, rust will have taken hold, even if you cant see it!!), you should first apply a coat of (preferably) zinc based etch primer, allow this to `flash-off` then primer or undercoat.

By "undercoat" I`m assuming you mean rubberised underbody protector? Obviously ensure this is the type you can paint over if you`re using it on the outer body!

Personally, I would etch, undercoat, then prime, if only because the primer will smooth the texture of the undercoat when painted over.

Another thing to remember is the more coats of paint you put on, the more liable to chipping it will become.

 

Great site, by the way, I stumbled across you guys by accident and used to own a 280ZX that I fully restored myself! Not the purest Z-car, but I was quite fond of it Have now progressed to a Nissan 200SX, so pretty loyal to the brand