smart repair for the door-skin?

[Waldi]

Hi Casar,

go for it, you want it:)

Cheers,

Waldi

[Casar66]

Nope Waldi!  The only thing I want is a nice (better nicer) door without an open heart surgery. I appreciate the advice here very much and I do not doubt them, but a try is just a try. So why not?

[Waldi]

Hi Casar,

my comment was not meant cynical, just saying “try it”.

Cheers,

Waldi

[Bfg]

.

I think we would all like to find an answer to repairing a cracked corner of steel with an adhesive.  As someone who had a long career in the automotive and marine industries I can assure you - we have indeed used all sorts of grp,  adhesives, bonding agents, and sealants  ..often to good effect.  But these were primarily used on fibreglass boats.  The problem with adhesives onto steel or aluminium (as some our larger yachts were constructed from) is five fold.    a.) poor adhesion to metal, even when an etching agent is used.  b.) shrinkage of the chemical adhesive as it cures ..makes it to want to peel off again.   c.) differences in elasticity between the metal and the adhesive.  Metal is ductile whereas the adhesives tend to be elastic in character (reinforced adhesives do help bridge these difference).  d.) differences in rates and degrees of thermal expansion and characteristics, ie. how they react when left in the sun  (thermal coefficients). One will expand more than the other and the heat may soften or otherwise weaken the interlayer bond.  And, e.) longevity before breakdown.  Many adhesives start to breakdown (at a chemical level) after a while. Rubber parts on a car for example tend to start to breakdown after three years and stressed items (like timing belts) are expected to be changed within 10 years. Similarly the effects of cyclic stressing will be different.       

Even if the bond doesn't break (because the adhesive is elastic and that adhesion is a good thickness and spread over a wide surface area) then you may still see the paint on the metal crack again, because it is less elastic and very thin.  And that joint will, by the nature of its design, continue to flex.  

I was faced with almost identical cracks on the driver's door of my previous classic car, even though that had quarter-light frames.  And like Stuart I thoroughly cleaned the area and welded it, with additional weld to fillet (round off) the inside hard corners.  However on that car I didn't have the benefit of a relief in the door skin panel, which might be painted to without necessitating respraying the whole door.  My decision to weld it though was down to the inaccessibility inside the door ..and into the tight corners of these cracked places - even with the glass and door mechanism removed and the door taken off and rotated or inverted. 

Going back to issue a.) adhesion to the metal.  For any adhesive to achieve a structural bond - that metal needs to be clean and preferable deeply scored as a key and acid cleaned with a chemical interlayer applied.  And that's not easy to do when you cannot even see into those corners with a mirror, and when there are panel overlaps which have been somewhat haphazardly brazed together.   IF it had been painted inside those places then it would be difficult to clean it out (without paint stripper seeping through the crack to damage the paint which you can see and wish to keep looking nice.  And if there was no paint, or else that has also cracked, along with the metal, then atmospheric humidity will most certainly have caused surface rust of the steel and oxidization of the brazing.   

5 hours ago, Casar66 said:

So why not?

Two fold.  1. although adhesives, perhaps reinforced with carbon or aramid fibre or additional metal, are nowadays very good ..especially in laboratory environments - the likelihood of structural success in these inaccessible corners, inside an old car's door, is rather slim.  To achieve it will take a great deal of time (..to fully strip the door out and in cleaning) and care (not to damage the door panel or get spills on your nice paintwork). And then, where the paint has cracked, would still need to be beautifully touched in.  I suspect (time wise and economically) you would be better off taking the car to a good-quality repair shop and have them do the repair and the top of the doors repainted down to the door's trim line.  

2.  if the adhesive doesn't work, or subsequently cracks again, then welding over the adhesive might prove more difficult, with perhaps it emitting toxic fumes &/or combustion - which may damage more of the surrounding area.            

These are just my thoughts.  But whichever way you decide to go -  I'm sure we all wish you success.  

Pete.

 

[Casar66]

Pete,

thanks for our thoughts, I agree mostly with all of them. Only 3 points which keep me attached to that idea: 

1. today many metall sheets of modern cars are glued not welded. So many different qualities of the sheets make them difficult to weld, eg. firm or high-strength sheets. So if they are glued means the adhesion on metallized is strong enough (modern cars do not seem to fall apart easily) and the difference in elastic should also be no matter.

But of course, these is an old car with old sheets and additionally I will not be able to copy the process of bonding like they do it in modern facilities. And my metall is painted. So the result will be not comparable. But neither the need of strength of my bond is approximately equal.

2. I have talked to an 3M engineer and to the adhesion-Specialist of Fraunhofer_Institute in Bremen ( I know him personally from another  medicine-project) . Both say that there is a chance that it will work (I described them my "problem" in detail. And they recommend me an adhesive special for painted sheets.

3. That is the point, where I am not afraid of and where I do not see a big Problem.: To fix it professional I have to remove the whole Brass from the sheet anyway. So if I have to do this, then the extra work removing the adhesive is not a big thing.

Summarized:

only little chance for success (but at least a chance)

rel. small money

rel. little extra work (but there is indeed only little space to work)

When it last 2 years before the paint in that area cracks again it is worth it. 

 

 

[Bfg]

.

I love it when people give things a try   ..after all that's how things change, often for the better.   In a few years time we might be using your formula to put our chassis back together again  (I'm note joking.  Most of the TR chassis is relatively lowly stressed).   My former boss,  car design engineer, and dear friend - Tony Stevens designed and built a road & race car in 1967 which was monocoque bonded together aluminium, so it's not a modern concept   ..just something most mechanics and/or government bureaucrats cannot get their heads around.  

For your door repairs, are you planning to use reinforcement like carbon or aramid cloth  ..either of which are readily available from any supplier of epoxies in boat-builting, or are your advisors saying the adhesive will be strong enough on its own ?

Tony Stevens  c.1967

Edited October 24, 2019 by Bfg
photo added

[keith1948]

Hello again Casar

I had a split near the hinge on the passenger door of the TR. I cleaned it up and removed a bit of paint around the split and cleaned the crack as best I could. Then I put a jack under the door to close up the crack and then welded it up from the outside. I use a low setting so not to blow a hole in the metal. Then I ground the weld flat and repainted it. That was a couple of years ago and the weld is still good. I was able to touch up the paint quite easily.

If your crack was not likely to be under stress then your idea of an adhesive would probably work. For example to fill a rust hole maybe. However the crack is caused by flexing of the door skin at that point. I would personally do as Roger suggested much earlier. Drill a small hole at the end of the crack. This stops the crack getting longer. Remove the rubber/plastic trim next to the crack. Protect the window glass with some plumbers cloth to prevent heat from welding breaking the window. Remove interior panel and have a water spray handy to put out any fire in case you set alight any wax inside the door. The crack is small so will not need prolonged welding. A couple of small tack welds to start with to stop any distortion. Then do a bit at a time allowing it to cool between welds. Use a small grinding wheel to remove excess and finish off with small files and emery paper. If you are careful and keep the repair as small as possible then it should be easy to touch up the paint.

Anyway - whichever way you choose good luck and let us all know how it works out.

Keith

p.s. I have a similar crack to yours on my drivers door so when I get around to welding it I will post the results

Edited October 24, 2019 by keith1948
added more info

[Casar66]

Bfg, the recommend adhesive ist 3M scotch weld DP 420 NS, NS stands for "Non Sag" means it is more a paste than liquid. The cfg-patch should increase the holding force against torsion, as they said. But the 3M guy was very optimistic that it will also work without the cfk patch. 

The guy at Fraunhofer said because of the pasty condition of the adhesive it could be better first just the adhesive then after curing (right word?) to laminate the cfk-patch with a more liquid epoxy on the already glued area. So first I will do one try just with the adhesive and the carbon fibre patch to get a lamination of the cfk startet with only the scotch weld adhesive. If this works I will do the same at the doors. if not, I do it like Fraunhofers expert said.

 

Keith, would be nice if you keep us posted here with photos before/during(/after the work at your door.