Bonnet pyrolysis?

Pim · Post by **Pim** » Tue Oct 20, 2015 9:01 am

I am preparing to have the tub pyrolysed (appr. 400 Celsius). Was planning on doing the bonnet too, but I am having second thoughts. The profiles inside the bonnet are glued on?

This will probably burn off and I am thinking this will be difficult to get in the right place again when assembling. Any advice?

TIA

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christopher storey · Post by **christopher storey** » Tue Oct 20, 2015 12:52 pm

You are absolutely correct - the heat will soften, and at that temp may actually set fire to, the sikaflex 221 which is the sealing adhesive used for all the bonnet parts . I would not do it, but would use paint stripper as a start, and finish with ground shell blasting which should not distort the panels

Pim · Post by **Pim** » Tue Oct 20, 2015 1:50 pm

The process of pyrolysis is oxygen free I think, so no fire just temperature. All paint, Stone chip protection, kit, etc. Will all burn off.

One other idea was to scratch the contours in the underside of the bonnet. Think I'll skip that too. Tub is glass beaded afterwards, think I'll have the bonnet in paint stripper and then glass beaded, which should be fine since there is no stone chip protection etc. to remove, just the paint.

christopher storey · Post by **christopher storey** » Tue Oct 20, 2015 4:38 pm

Sikaflex 221 is isocyanate based , so has its own oxygen supply in the cured compound I think, and the fumes would be very very dangerous. In any event , paint cannot burn without oxygen ! I would be very careful with glass beading, you are quite likely to distort the panels, and once deformed there is no way to restore the contour. The experienced restorers are all very reluctant to media blast bonnets for this reason

Pim · Post by **Pim** » Tue Oct 20, 2015 4:57 pm

As I understand the process, the fumes will go through an "afterburner" to minimise environmental impact. Don't know what 800 Celsius does to cyanate, but trust it to be OK. Anyway, bonnet is not going in. The frames stay out as well as I am not sure the brazing will stand the heat.

As for the bonnet, the blaster says it is very mild but I will monitor closely. Will ask him for possibility of using soda.

JagWaugh · Post by **JagWaugh** » Tue Oct 20, 2015 6:09 pm

Pim,
Have you considered having the shell/bonnet dipped. The place that I had the whole shell of my saloon dipped at uses a glycol solution to remove the paint, then phosphoric acid to derust. then I had them apply an epoxy wash primer. Pyrolysis may well remove the paint, but even if you have it media blasted afterwards that will only remove rust from the _visible_ surface (if at all).

Andrew

64etype · Post by **64etype** » Tue Oct 20, 2015 6:50 pm

JagWaugh wrote:Pim,
but even if you have it media blasted afterwards that will only remove rust from the _visible_ surface (if at all).

Andrew

On close inspection of the bonnet on my '64 I found a loose duct mounting bracket with rust under it. Decided to remove all of them. Found rust under many of them, even though they appeared to be "securely bonded". Careful measurements from known reference points along with lots of photos made proper placement quite easy. The rebonding process was, however, very time consuming as I first epoxy primed the entire underside of the bonnet, as well as all of the brackets (with particular attention paid to the back/bond side). Then all of the areas where the Sikaflex would be applied were masked off and scuffed. Then the mounting flanges were glued in place (using the masking tape as a guide). This was all done within the "open" window for the epoxy paint so that I could get a chemical bond between the epoxy and the finish paint. Of course all of the metal work had been completed. I built a raised cradle/table for the purpose. There are a couple of strings about this on J-L. Clive Wilkinson has archived some helpful photos.

Pim · Post by **Pim** » Tue Oct 20, 2015 6:58 pm

Hi Andrew,

I have considered dipping, but decided against it, I am concerned about acid staying in the nooks and crannies and coming out after painting.

My car does not have a lot of rust so that is not my major concern. I have inspected the inside of the tub as far as accessible with an endoscope. Just a bit of surface rust, if any.

norman m. macleod · Post by **norman m. macleod** » Tue Oct 20, 2015 7:13 pm

If the bonnet still has the original bonding compound, it cannot be Sikaflex 221, as it was only introduced in the early 80's.......?
Just for the record.........
Regards,
Norman

Pim · Post by **Pim** » Tue Oct 20, 2015 7:47 pm

Looks original to me, as under the paint that I think to be original. Underside of the bonnet has not been reprayed.

norman m. macleod · Post by **norman m. macleod** » Tue Oct 20, 2015 9:48 pm

Hi Pim,
It also occurs to me (re the tub), that at 400 deg C any lead loading present will be melting.....? Whole thing is starting to sound a bit risky, with minimum pros, and many cons.....Soda blasting has no heating problems on large thin sections, but whilst it will remove paint and filler, it is not very effective on rust. My local blast-man (an ex-soda blaster) has done 2 tubs and bonnets for me without any distortion whatsoever. He uses some ultra-fine media/process which he won't discuss, but results are outstanding. About $1500 for body and tub, including immediate two-pack epoxy primer spray. Regrettably a bit distant for European owners,,,, :) :) ......!!
Best regards,
Norman.

christopher storey · Post by **christopher storey** » Wed Oct 21, 2015 9:46 am

Norman : sikaflex 221 was the original bonding agent from the introduction of the car in 1961

norman m. macleod · Post by **norman m. macleod** » Wed Oct 21, 2015 12:01 pm

Hi Christopher,
Well, the Sika AG info site agrees with me, with all due respect. I use Sikaflex 221 to re-assemble bonnet components, and in other applications. Wonderful stuff........... However, the 2 bonnets I have dismantled (original assemblies) 1 x 1967 and 1 x 1969, had some black sealer that, for all the world looked like a bituminous mastic (and smelled like it when heated with a hot air gun to soften it...... :( :( )
Best regards,
Norman.

christopher storey · Post by **christopher storey** » Wed Oct 21, 2015 2:58 pm

Norman : I think you will find that what has happened is that the product was reformulated at some stage in the 1980s, to reduce the isocyanate exposure . We had in the UK a number of fatal accidents as a result of isocyanate sensitisation, with several of which I was concerned professionally, which led to wholesale changes in the regulations. Certainly, those who were at Abbey panels , who have lectured to me, are quite certain that it was sikaflex 221 that was used, and since they made, as far as I know, every E type bonnet, I imagine they have direct knowledge of the matter

Having looked at the promotional blurb which presumably is the source you refer to , I am even more unconvinced. A the lack of precision in a statement like " in the early 1980s" does not give any confidence, and I merely make two points 1. There are probably few if any people at Sika who were around in the early 1980s 2. There will be even fewer people who remember what was produced in 1961

There we are - but does it matter ? Sikaflex 221 is certainly what should be used

Pim · Post by **Pim** » Wed Oct 21, 2015 3:10 pm

Well, car is at the bakery without the bonnet so rien ne va plus.

Will check the bonding stuff for colour, seem to remember grey stuff but nit sure.

norman m. macleod · Post by **norman m. macleod** » Wed Oct 21, 2015 6:29 pm

Hi again Christopher,
Thanks for the clarification, you obviously know much more than I do about the history of the stuff.....! :) Perhaps one should not put too much faith in the likes of the Sika range description one trips over on the website.....

In any event, we are agreed it is the perfect stuff for the job.
Thanks again, and
Best regards,
Norman

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