Galvanic corrosion in a CF roof. Opinions Please.

General Composites Discussions Composites Talk
#1

I am looking at a 1971 Porsche 911 as a long term restoration project.

This one has a sunroof that for a number of reasons, I want to get rid of.

The biggest being the removal of some 60 lbs from the highest point of the car. (the sunroof panel is very heavy)

The best solution I could think of was to remove the sunroof and hardware, cut the hole somewhat larger (75% of the roof area) and form a Carbon Fiber plug with a flange to fit from underneath. After bonding, a few layers of high build primer and then its off to paint.

I have 2 concerns with this concept.

  1. Is there a possibility of galvanic corrosion with the steel body. Will the resin block the electron flow or would a coat of self etching primer do the same. If primer is good, does it then become the weak point of the bond?
    Do I need a layer of fiber glass between the CF and steel?
    Or will the paint seal it well enough to prevent this problem.

I am planing to keep this car for decades, so it is important to get to right.

  1. Thermal expansion.
    Am I going to have issues with paint cracking as a result of the different coefficients?

Thanks in advance to all the experts out there.

Rob

#2

If you are going to paint it use e-glass, or at least glass on the outer layers. This will act as an insulator and glass expands around the same as steel or at least it will be pushed around by it so shouldn’t crack. But in my experience it’s the filler or bog that shrinks and cracks so try and minimize the amount used.

Another thing to keep in mind is how hot the roof will get especially if you paint it a dark colour. Make sure the resin has a high enough Tg, otherwise you could find your roof sagging when left outside on a hot day. It might need to be postcured in order to reach the required Tg, which you will need to do off the car.

#3

What kind of 911 is it? S? T? E?

There are a couple of things that cross my mind:

-carbon in contact with steel or any other metal, in a (potentially) wet environment should only be done with a layer of E glass in between. Also possible rivets should be isolated.

-bonding carbon with steel is perfectly possible, using either a methacrylate glue (less surface prep) or an epoxy glue.

-the different thermal expansion will very likely cause a seam to be visible, perhaps not the first year, but potentially afterwards.

-Especially pre-1973 Porsches 911S will become very sought after in the near future. IF yours is one, and is relatively original, I would do my best to at least conserve the authenticity.

-A sunroof (original or later installed) indeed might impair drivability, and penalties might be low if exchanged for a fixed roof.

All in all if it was my car, I would install a metal sheet roof, and if braces have been cut, reinstall these as well. In that case you might find your luck at www.metalmeet.com, which is a metal shaping forum.

My father and uncle drive rallyes and hill climbs in a 911RS 1973. This was a 911S originally, but in 1974 has been converted to RS specs by Porsche AG, and paperwork and files have been changed accordingly. Someday during its life the fuelpump has been exchanged for a 911S unit, which basicly made the car undrivable in high revs. (but cutting on consumption). It also received a bent rear frame at some time.
My uncle corrected these problems (took some time, especially the fuel pump, which needed modyfying by Porsche Classic AG), installed an RS racing interior, and now they have a lot of fun.

Actually, they are now looking for a 1974-1981 Porsche 911 3 liter, to restore and convert to RSR specs. If you know of any…

#4

Thanks very much for the valid points.

The resin I was planning is good to 180 C which should be fine.

The car I am looking at is a T. It has been converted to look like a SC which I will be reversing.

The longhood 911s are indeed becoming very collectible, but this one is for me. Resale is not important, only getting it right.

I think making and bonding a panel is the right choice for a track car but the difference in expansion rules it out for a street car.

There is a gent in the UK who makes a steel panel on an English wheel (pun not intended), I just might give it a go.

Would love to see a pic of the RS though!!

Cheers and thanks very much.

Rob

#5

Wow! Neat car and the idea of it only going UP in value as time goes by would make me SERIOUSLY consider doing anything to modify it from the way it is. Perhaps there is some way to install a painted aluminum panel or CF panel in place of the glass? You could have some headliner material professionally installed to nearly match the interior of the car. I know you mentioned the car won’t be sold for 300 years or so, but sooner or later your great-great grandson or daughter is going to get rid of it and they could have gotten more out of it “If only grandpa hadn’t cut the roof to save a few pounds…” You could always keep the glass and related components in a safe area so the car could be put back to original if the time ever comes…? Just a thought. =)

#6

:smiley:

here is my project… recognizable?

#7

Definately recognisable. Tell us more…

#8

Herman

Those are fantastic photos. The first one needs to be hung on your wall.

Dallas

Very valid points indeed. I will keep all the components with the car just in case. Having said that, if my great great grand daughter has any of my DNA she won’t sell either.:smiley:

The car in question has already been modified to look like a later SC.

Worst of both worlds really. If you wanted an SC, you would buy one and get a galvanized car with a bigger engine.

This leaves me with a “backdate project”. Ending up where the car originally started.

Lets me take some creative liberties though.

Max Du Sabb

You really need to tell us what you are planning. It looks VERY cool.

Cheers and thanks guys.

Rob

#9

:amazed::amazed:You bet I recognize the car…I’m here to learn how to do what you’ve just done. Please share any photos and procedures you’ve perfected to produce such a nice looking part.

#10

I produce a replacement carbon roof for the classic Mini and haven’t had any problems with corrosion at the bonding area. What we’ve done is remove all but a 1" lip all around the roof line and then use epoxy panel adhesive to attach the new roof and then seal the edge. I do use some kevlar in the layup which you could use on the inside surface to help prevent corrosion.
For your application, could you just replace the sun roof panel with carbon and remove any mechanism that could add weight? Or if you really think it best to cut the hole larger, what about just removing the majority of the roof. I add stiffening ribs to the Mini roof and the driver of one of the cars that is regularly raced feels the carbon panel is stronger and definitely quieter than the stock roof.

#11

[COLOR=black][FONT=Times New Roman]I am still in the composite learning curve as an amateur.
I made a copy of the rear fenders. The intention is to replace all removable parts into CF to make a “light-11”. (911 SC)
photos: Mold + Weight in KG. steel vs CF
(Don’t have any more photos on my laptop, I am traveling)
[/COLOR][/FONT]

#12

Please post more when you have a chance. What technique did you use to produce the parts (infusion…wet layup?)…looks great.

#13

Please post more, what a great project.

I bought the 911T last night. So I have some decisions.

Cheers

#14

Last night? We deserve pictures!!! :slight_smile:

#15

At some point, it was modified to look like an 80’s SC but it is a very straight and clean car.

A good start.:smiley:

Cheers

Rob

#16

[FONT=Times New Roman]CongratulationsRob. [/FONT]
[FONT=Calibri]Is it your first 911? How is the engine? (Not in on the photo, is it?)[/FONT]
[FONT=Times New Roman]Is weight the only reason you want to remove the sunroof? [/FONT]
[FONT=Calibri]Please post more photo’s of your car.[/FONT]

[FONT=Times New Roman]Twenty years ago I glued a glass fiber roof on my 911 without any problems (and it is still on)[/FONT]
[FONT=Times New Roman]@ Canuck, parts are done by infusion. [/FONT]
[FONT=Calibri]Photo: rear seats look strange but it’s necessary for the adjustable rear suspension and G50 gearbox. (Don’t worry the definitive one will be from CF. )[/FONT]

#17

Nice car, but ditch the wheels… :slight_smile: