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Thread: Using black silicone over rust repairs?

  1. #1
    Join Date
    Jul 2010
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    14

    Default Using black silicone over rust repairs?

    A quick question for all you undercoating professionals.

    My 14 year old VW restore project has some minor spots of surface rust on the underbody sheet metal. Nothing major. Most of the spots are about the size of a dime or smaller, and are underneath the original factory undercoating (which I assume is some sort of urethane.)

    In order to get rid of the rust, I have been scraping away the original undercoating to get at the metal, then grinding the rust off with a wire wheel, then treating the spot of rust with phosphoric acid (to eradicate whatever is left of the rust), then spraying a few shots of rust reformer on the spot. When everything is all dry, usually after two or three days, I then respray the area with 3M rubberized undercoating to permanently cover up the treated spot.

    However, yesterday, I decided to try something a little different for a ďspotĒ undercoating. What I did was apply a dime-sized portion of Permatex black silicone adhesive sealant to the treated rust spot, then I spread it around with my finger until it completely covered the spot and blended in with the factory undercoating surrounding it. I let it dry for a few days, and today, it seems to be tight as a drum. Itís pliable, yet firm, and stuck to the underbody like a piece of chewed up gum.

    My question is, has anyone out there ever used black silicone adhesive sealant to do spot repairs on the original undercoating? And what kind of success have you had with it? Will it stay adhered to the vehicle over the years, and will it provide adequate protection from the elements, primarily water and moisture? Iím not talking about doing the entire underbody with silicone, just using it to cover up those spots of rust that Iíve repaired.

    The 3M rubberized undercoating seems to work okay, but it never dries. Itís always sticky. Maybe thatís okay and Iím just being impatient waiting for it to dry. But while doing these minor rust repairs, I was hoping for some sort of undercoating that would dry and look just like the factory undercoating. So far, the Permatex silicone seems to do the trick (see attached photo.) Iíve driven all over town with it on the bottom of my car and it hasnít moved an inch. But that may not mean anything.

    Iíd appreciate any comments and suggestions from the professionals on this forum before I go sticking any more silicone on top of all my underbody rust repairs. Is there something else that's better?

    Thanks so much guys. I really appreciate it.
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  2. #2
    Join Date
    Nov 2005
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    27,762

    Default

    Rubberized undercoat and seam sealer are a lot alike, they (like the original undercoat) tend to crack and peel as they age then they trap moisture underneath and help create the problem they were used to solve. It's best to use a "good" rust converter and a rust preventive paint that will bite into the surface and not allow moisture to get to the metal. I've been doing this type of work for 45 years and the best products I've found so far are Picklex 20 to convert the rust then Zero Rust to coat the surface after it's prepped properly. You should GRIND away the undercoating so that you are left with the rusted spot and clean metal around it then apply the rust converter, allow it to dry, scuff it with sandpaper and apply several coats of Zero Rust allowing it to flash between coats.

  3. #3
    Join Date
    Jul 2010
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    14

    Default

    Thanks Len. I appreciate it.

  4. #4
    Join Date
    Mar 2008
    Posts
    140

    Default

    I'm not an expert, but I did a quicky VW floor panel job years ago. And it was rusted so bad in areas that I could not weld all the way around, so I did large tack welds on most of it. Then used an aluminum like metalic tape all over the seam, then painted with a brush seam sealer all over that. Then sprayed zero rust over it all. Looked good.
    The silicone will not last, it doesn't even work all that good for its intended purpose. I very rarely use it. I'm no body person or painter specialist. But am an Engine specialist.

  5. #5
    Join Date
    Jul 2010
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    Default

    Well, fortunately, my VW doesn't have the kind of rust you're talking about. The undercarriage/floorpan is in very nice condition. The only problem is that here and there are some rust spots coming through the factory undercoating (see pic.) As Len suggested, I'm grinding away the rubberized undercoating to get at the rusty metal, then grinding the rust away, chemically treating it, etc. I've got that part down pat.

    My problem is figuring out what to replace the original factory undercoating with in just the spots where I've repaired the rust. Like I said, I was using 3M rubberized undercoating. It seems to work, but it's never dries. It's always sticky. That's why I tried the black silicone...it dries rubbery hard.

    Ideally, I wish I knew what VW uses for their factory undercoating. It appears to be rubber, or urethane, or something along those lines. If they sold it in a bottle, that would be perfect. Anyway, in a nutshell, I'm just trying to replace the chunk of rubber undercoating I had to grind off in order to fix the rust, with something comparable. I don't want to leave the metal of the underbody exposed.
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  6. #6
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    Jul 2010
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    Here is the answer to my own question...Wurth Stone Guard Spray. Can you get this stuff Len? If so, I'd be interested in buying some from you. If not, I can shop www.goodspeedmotoring.com but I'd rather give you the sale if possible. It's comes in gray and black, and is supposed to exactly mimic the factory undercoating of most vehicles.

  7. #7
    Join Date
    Nov 2005
    Posts
    27,762

    Default


    Quote Originally Posted by LambertJohn View Post
    Here is the answer to my own question...Wurth Stone Guard Spray. Can you get this stuff Len? If so, I'd be interested in buying some from you. If not, I can shop www.goodspeedmotoring.com but I'd rather give you the sale if possible. It's comes in gray and black, and is supposed to exactly mimic the factory undercoating of most vehicles.
    Isn't that the textured material that is applied to the lower panels on the EXTERIOR of the vehicle then painted? If that's what you're looking for I'd recommend A Sikkens product called OTO Body Coat. This is a water based product that is applied to a painted or primed surface then painted. I don't think it's a not a rubberized undercoats like under the vehicle.

    If you want OTO Body Coat you'll need a shutz gun for the application. I have these products.

    If the damage is UNDER the car then you'll probably want to prime or use Zero Rust over the treated metal then shoot it with ruberized undercoat aerosol to help match the existing texture.

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