Firewall Sealant

[Floyd B.]

On my '73 there appears to be the remnants of a black tar sealant on the firewall and inside the wiper compartment in front of the windshield. It has badly deteriorated and has dried and cracked. Much of the material has fallen out.

It was used to seal the seams where the AC/Heater (passenger side) and power brake booster (driver side) join the firewall. It was also used to completely cover the wiper pivot brackets that are bolted just under the windshield. This also appears to be the same material that is used to seal the seams in the wheel wells.

The sealant is described in the Tech Info Manual & Judging Guide as:

"This sealant is a thick black asphalt-like material that often is not carefully applied.", Pg 90.

I use to work in the Ft. Wayne and Pontiac GMC truck assembly plants and remember that a similar black tar sealant was used on the final assembly line (i.e. after the body emerged from the paint shop oven). This sealant looked like roof repair sealant that you would buy at Home Depot and was applied to seams that could be exposed to corrosive liquids such as water mixed with road salt. However, I cannot find a similar material being sold in any of my Corvette restoration/parts catalogs. I have several questions pertaining to this sealant:

- Can this sealant be purchased anywhere? If not, does a material like black tar roof sealant serve as a replacement? (i.e. the black roof sealant that is sold in caulk tubes at any home repair/improvement store)
- Does anyone know exactly where, how much, and how thick this sealant should be applied in the engine compartment and wheel wells of a '73? What are the judges looking for here?
- Is the sealant applied before or after the paint is applied to the engine compartment and inside the wheel wells?

Thanx,

-Floyd-

[Chuck S.]

Re: Firewall Sealant

Floyd, I believe the products used in the wiper plenum, on the dash panel (firewall), and in the wheelwells were originally three different materials.

These materials, like most of the adhesives, sealants, lubricants, and the other goop used to put these old cars together went the way of dinosaurs long ago, and have been replaced by new products and techniques. The restorer is stuck with trying to find something that functions as well, is available to the public, and is physically undetectable from the original material.

Black tar roof sealant: I looked at this product and discarded the idea because I believe that its consistency (pasty) in caulking gun tubes is too thick to brush on (like in the wiper plenum) or to make original-looking runs down the firewall. If you try it, let us know how it comes out. You may be able to thin it using mineral spirits (paint thinner).

Here in the southwest, there is product sold in building supply stores for evaporative coolers that very closely resembles the sealer used for C3 plenums and firewall penetrations. It is an asphalt based product called "submarine sealer" that is the consistency of paint and just right for the wiper plenum; in my estimation, a perfect match. Leave the lid off a few days to vent the light fractions out, and it becomes the perfect sealant for the firewall penetrations and will provide all kinds of original looking runs, but not all the way to the floor. Shhhh, I have never told this to anyone else. Some enterprising Corvette supplier could buy this stuff for next to nothing, and sell it for...what? $37.50 a quart?

I don't think the same material was applied in the wheel wells of Corvettes. I think the wheel wells were only undercoated using a different product. In any event, undercoating materials are still available in forms that will replicate the original.

Regarding painting, if the plenum sealant was applied in the body shop as I think it might have been, it would have been painted black after application. Otherwise, it would have been applied after black out paint and before the wiper installation. Logically, the firewall sealer was applied AFTER black out, since the black out was done before any installation of cables, wires, and their seals were installed in the firewall. I think the wheel wheels were blacked out before undercoating, but it was hard to tell on my car.

[Floyd B.]

Re: Firewall Sealant

Thanx Chuck. I'm new to this hobby, so I really appreciate the detail in your response. Since you mentioned the black-out paint. Could you recommend a good paint? The NCRS Judging Guide describes it as semi-gloss black. Would any semi-gloss black do? (for example, rustoleum).

Also, I forgot to ask about one other item: The upper A-arms are covered by flaps made of a heavy tar-paper like material that is held in place with 15 - 20 plastic rivets. The material appears to be in pretty good shape, but could use a "freshing up." The Judging Guide seems to indicate that these would have received the same treatment as the rest of the engine compartment (i.e. sprayed with black-out paint. Any suggestions?

Perhaps you and I should start a "high-quality restorations sealant supply business." ;-)

[Chuck S.]

Re: Firewall Sealant

Semi-gloss black: For a complete restoration, the purist would be able to use GM Part No 1050104 (one gallon size), but don't expect to buy a single gallon from your Chevy dealer (full carton minimum to my knowledge). For a single gallon try Quanta Products. If you are only doing a small area resto, I understand Krylon 1613?, semi-gloss black is a good match and readily available. Others swear by John Deere "Blitz Black", but be sure you get the right gloss level; I think it comes in several glosses.

If you're hoping that the dust shields are painted with a thick coat of semi-gloss black, they aren't; natural rubber finish. :-) What did you see in the TIM&JG that made you think that? "Original dust shields are made of rubber and have no part number or country of origin stamped on them." This implies to me that they are unpainted. Good reproductions are available if you find that replacement is the desireable path.

[Floyd B.]

Re: Firewall Sealant

Chuck,

Thanx again. I didn't really see anything definitive in the TIM&JG, that's why I asked. Just the fact that it describes the inside of the wheel wells as painted black-out and doesn't mention if the dust shields were over-sprayed or not. I'm at the office on my lunch hour, so I don't have an assembly manual handy to see if they are installed before or after the engine compartment paint process.

I'm starting to wonder if mine are original. They don't seem like rubber. More like a heavy paper fiber material. But I don't know what happens to rubber after 31 years.

I'd just like to "spruce" them up a bit for now and come back to them later. This car has too many other (and more pressing) needs right now. I wonder if a little black shoe polish would do the trick? I suppose I could buy new ones now. My wife is really starting to hate this hobby. :-)

[Chuck S.]

Re: Firewall Sealant

Floyd, I believe someone knowledgeable once told us that the black out painting was done either at the end of body painting or at the beginning of the assembly line. I recently rediscovered that black out occurred AFTER the body was painted (it's in the TIM&JG). Therefore, black out painting would have been done before almost any assembly of parts to the painted fiberglass body. If your car is fairly original, it will tell you a lot of this kind of information if you are observant and think about what you are seeing.

Your dust shields sound original. If you are going to get the car judged, I would scrub them clean using Simple Green or dish washing liquid and leave them as clean as I could. If they have been painted (latter day detailing), you might try using some lacquer thinner to remove any light overspray. If they have a fairly heavy coating, I don't need to tell you that lacquer thinner could damage that old rubber if you scrub on them much.

In judging, it will be better to have the originals clean, than to try and spruce them up by painting them. Your dust shields will be judged for condition if they are the originals, and in as good condition as you say, deductions will probably be minimal. If you aren't getting the car judged, then paint'em or not as you prefer.

Believe It or Don't: Your wife is hardly a pioneer when it comes to spouses hating the Corvette hobby. Clint Eastwood/Dirty Harry: "A man has got to know his limitations." :-)

[Floyd B.]

Re: Firewall Sealant

Chuck,

I think you have come up with another product to go with our "Firewall Sealant" - we can repackage household cleaning fluid as "Rubber Restorer" at...say... $10.95 for a 10oz spray bottle. :-D

BTW, my '73 appears to be quite original, but it has had some restoration work done: interior door panels, carpet, and seat upholstery replaced with replicas, California emissions stripped, new (and not so original exhaust) etc., but the one that can throw me once in awhile is the fact that it was repainted with the original color (or a close proximity), this tends to create body paint over-spray on top of other over-spray. But you are correct. I need to learn to rely on my engineering background, experience in assembly plants and the logical assembly sequence a bit more.

[Gary S.]

Re: Firewall Sealant

Floyd, others have recommended John Deere "Blitz Black" paint in semi-gloss and I have had good luck with regular old Krylon 1613 semi-gloss. John Deere dealers in Pittsburgh are few and far between and everybody sells Krylon.
Gary