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Dealing With Heater Box Rust


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I pulled my heater box for the first time today. Not surprisingly there appears to be some rust back there behind the wood/sound deadening stuff. Mostly, however, there is a bunch of tar. 


I do not have a garage to work in and do not have a bunch of chemicals lying around. I have not really dealt with rust before. But of course I am not going to put the heater box back in without addressing this.


I am hoping for some advice as to the best and simplest ways to deal with this (and what to buy).


I assume I will have to remove the sticky stuff before dealing with the rust. Searches of the FAQ mostly deal with removing the whole carpet for this. I take it I need to be chiseling out the tar? (dry ice seems like it would be difficult to work with under the dash). Is there a chemical well-suited to this task?


After removing the tar, I need to sand away the rust, and then spray with some rust inhibitor. I have some Rustoleom Underbody Rubberized spray, would this work here or not? POR-15? Then do I have to paint it to further protect from rust or is that enough?


Any particular advice on dealing with the surface rust on the hood side (underside of cowl)?


I've been searching the FAQ all day reading up on heater issues, hopefully I won't have too many more questions to post as I go through this.


Here are some pictures of what I am talking about, though I think the tacky stuff looks more like rust in the pictures.






Also, bonus points if anyone can tell me what those two stuffed gaskets are from (removed A/C?)

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The stuffed gaskets do look like where old A/C lines may have been routed.  That is the general area where they are usually found.


For the rust, IMO you cannot do better than POR-15 products.  Rustoleum is worthless as the rust will come back relatively soon, if not sooner.  The Rustoleum undercoating will seal the area and allow the rust to go into overdrive under the undercoating.


The first step is to clean and degrease the entire area.  On the heater box, you will first have to remove the tar/undercoating material.  First scrape off as much as you can, then use a heat gun to heat up the remaining tar/undercoating and use a solvent, a brush (stiff toothbrush works well) and rags to clean off whatever remains.  After you have removed the tar, you have to degrease the area to prepare it for painting.  You can use a product called Prep-Sol or another called Ultra-Klean, both available at body shop paint stores.  POR-15 also makes a good degreaser that they used to call Marine Clean and they now sometimes label it "Cleaner and Degreaser".


Once you have cleaned and degreased, you can use another POR-15 product called "Metal Prep" (now sometimes labeled "Metal Ready").  You spray or brush that stuff on, let is sit WET for 10 to 30 minutes depending on whether you are trying to remove rust or just "convert" the rust, making sure you first have gotten rid of any flaking or loose rust by brushing it with a wire brush or a wire wheel on a drill.  Then you have to rinse off the Metal Ready and dry the area thoroughly.  Here, after you have dried everything with clean rags, you can  go over the area with a heat gun to make sure it is perfectly dry.


Now you can brush on the POR-15.  It flows out nicely and will not leave brush marks.  I prefer the gloss black as I have found it to be their most effective anti-rust formula.  After that, since these areas will not be exposed to UV rays, there is no absolute need to topcoat it.  If you want to topcoat it, you can use any paint or color coat that you want or you can continue to use the POR-15 products, such as the "Tie-Coat Primer" and "Black Kote".


I have no affiliation with POR-15 but I can honestly say that the stuff I have used the most over the last 25 years or so (the gloss black original formula) is bullet proof.  If you prepare the area properly, the rust will not return.


BTW, make sure you use nitrile gloves when handling these products.  The POR-15, once it cures, cannot be removed by solvent, only mechanical means (i.e., you have to grind it or sand it off your skin).


From the photos you have provided, it does not look like you have any flaking rust and you do not have to remove all of the rust completely before using POR-15.


You can use the same procedure for the area under the hood/ underside of cowl, but you should topcoat it to prevent UV damage to the POR-15 coating.


Regards, Maurice.

Edited by schoir
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Maurice is correct - those two empty grommits with foam are for the previous A/C lines.


Clean up the rusty areas and use your favorite rust preventative product to keep it from coming back.  The original foam gasket for the heater box-to-body is a trouble spot for these cars.


While you have the heater box out - NOW is the perfect time to remove and refurbish the wiper motor and linkage.  It is MUCH easier while the fan assembly is out.

Jim Gerock

Ruby Red 73tii built 5/30/73 "Celeste"

Riviera 69 2002 built 5/30/69 "Oscar"

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Maurice, thank you so much for the detailed instructions, that is just what I needed. So rust converter is a legitimate product? It had always kind of assumed it was bs. I'm going to try and add a heat gun to my harbor freight order now.


Jgerock, I'll get searching on wiper linkage, I already have the motor halfway removed (to get to the drivers side heater hose clamp for the heater).


Thanks again.

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Anytime!  Since you are getting a heat gun, you can also use it to make sure the surfaces are absolutely dry after you have used the Metal-Ready and rinsed with water.  The Metal-Ready etches the metal and gives it "tooth" that the POR-15 can adhere to really well.


Although prepping rusted metal for paint is a royal PITA, you will get a really good feeling when it's all done and you know that the rust is not coming back.


If you get a chance, post some "after" photos.


Regards, Maurice.

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