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Author Topic: Galvanised subframe  (Read 9599 times)
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« on: 30 March, 2007, 08:02:05 AM »

Dropped the engine and subframe out yesterday and found some poor spots.After shotblasting and welding I was thinking of getting it galvanised rather than painting. Seems logical,but has anyone any thoughts as to why not?
« Reply #1 on: 30 March, 2007, 09:23:43 AM »

Some years ago I looked into this and was advised not to get the subframe galvanised using the 'hot bath' method for a number of reasons:-
1, As it is almost impossible to ensure that all the cavities and box sections will be completely clean there is now way to make sure that the zinc will adhere to the interior surfaces and therefore will not 'protect' the steel.
2, To get a good 'bond' the metal needs to be shot blasted and you cannot do that for the cavities and box sections and therefore as in 1 above you will not get the required protection.
3, The process involves very high temperatures and this can lead to twisting and distortion of the frame, unless bolted to a jig.

I was advised to use (and did) the 'hot spray' method, the subframe is shot blasted, all the threaded holes had bolts inserted to prevent the threads from being coated and then it is spayed with hot zinc.

The resultant finish is quite rough but this acts as a good key for primers (I used No 1) and a top coat.

When I was first put onto this method by a fellow car restorer I was concerned as to how durable the finish would be, but he showed me an item which he had done and then he hit it with a hammer, the steel dented but the coating stayed in place.

I was so impressed, that no only did I have the subframe done, but all the suspension components. The advantage was that even if the paint chipped there was no corrosion so it was a simple matter of a quick clean and touch up with a bit of paint each year.

Before I refitted the subframe I stood it out in the sun for a few hours to get it hot and then filled it with large quantites of Waxoil to protect all the box sections and cavities. I hope you find this useful and that you will be joing the LMC soon.

Rebel Poster
Posts: 980

« Reply #2 on: 30 March, 2007, 09:33:09 AM »

Cannot think of any reason why not. I got mine galvanised using the dip method after being shot blasted- all the steel subframe components including the transverse spring U-section. I had no problems with distortion - however I do agree with Chris that you should assume the insides of the box sections are not fully protected. When it has been done and before it goes for painting (I think you should paint it - I had mine stove enamaled) you will need to go down all the threaded holes with a tap (even if they are blanked off- as you may get some flash around the top) and do a dry run of the basic assembly as you will probably find some flash that will require removing around bolt holes or the various edges. Then give the box sections a good blast of Dinatrol or similar.
If you are considering getting the aluminium "done up" they willl need bead blasting - anything else is too aggressive.
It took me about three months to complete my subframe. Relatively straight foward if:
a) you have the correct tools for the hubs
b) you are carefull with the removal and installation of the spring

All the best

1969 Fanalone, Mazda RX-8, Fiat Multipla
Rebel Poster
Posts: 723

« Reply #3 on: 30 March, 2007, 10:06:19 AM »

I can't see any real disadvantages and truthfully it will be a million times better than the original.Don't forget though the subframe is only as good as the points you attach it to.A galvanised one that rocks due to rotten mounting boxes is also no big help.
 1)Potential concerns;
Tolerances; galvanising is usually +/- 70 microns.Components with captive nuts can then be a nuisance (retapping is quickly done).
Galvanising is only as good as the surface preparation. Transitions (in fact everything) need to be very well prepared, cleaned, fluxed etc. Inside sections cannot be done will have to use dinitrol etc inside afterwards.

Hot bath galvanising  (or rather the temperature it is done at 450c+ ) can in some cases lead to embrittlement and weakening of the metal due to 2 or 3 different causes .(bonding of hydrogen is one). May have to watch the it cosmetic or really major structural sections?Primarily associated as a problem with stainless or specialist steel products and usually at points that have been cold folded and/or are thin . Probably not too big a problem in this case I think.

2)Potential advantage;
possibly better than my paintbrushed black hammerite. But having said that , 13 years on there's still no rust... but I think that's the waxoil..

 I would have it blasted, repaired and then paint with a hard powdercoating and waxoil every couple of years . keep it clean and you'll have no problems.
« Reply #4 on: 30 March, 2007, 10:19:26 AM »

Wow,lots of food for thought there. Agree galvanising is only as good as the preparation and its impossible to clean internally,therefore protection is external only,however I'm inclined not to worry too much  about possible  distortion. I'l get it shot blasted first and see what condition the steel is in before deciding on dip or  spray. Thanks .
Senior Member
Posts: 136

« Reply #5 on: 30 March, 2007, 07:57:24 PM »

Yes Kim, lots of sound advice above, I have treated several Fulvia,s to various plating methods over the years and can wholeheartedly recommend one :- Metal (Galvanize) Spraying. I did get one subframe electroplated, but, unless it was almost like new (so defeats the object!) I honestly don't think its worth this extra effort. As you probably know, unless the item to be plated is all good steel, the process of "attracting" the plating won't work and even if it IS 100% good steel, you will not be able to satisfactorily plate the box sections internally, contrary to popular belief. Additionally, due to the whole electroplating process, you have the possibility of acid being trapped within the box sections, even after a thorough water dunking*.  The good thing about metal spraying is that you can get a generous amount of zinc onto your item, the fact that it is applied hot acts as a good "sealer" to all of the subframe, including any slightly corroded areas, there is no problem with acid leaching into impossible to reach areas. Finally, last one I had done was almost half the cost of "plating" (was about 12 years ago though). Yes, there are a few tolerance issues post spraying, but nothing serious. Whatever process you settle on, don't forget to fit bolts (I copaslipped mine!) into the seven captive nuts. *If I were to get a subframe electroplated now, I would most certainly immerse the whole thing in a bath of oil and leave it for some time before using it.  Best Regards Chris
« Last Edit: 01 April, 2007, 10:24:15 PM by chris » Logged
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