Just Some Info/to Be Aware Of - Rear Subframe

Powercoating does not bond to the surface, it melts to form a case around the metal. As soon as you bend the metal or crack the powdercoating you will trap moisture between the powdercoating and the surface and accelerate corrosion. Epoxy forms a chemical bond to the surface so it is less likely to crack when the metal is bent, and will protect the rest of the surface even if damaged in one area.

https://www.bilthamb...gs/epoxy-mastic

You could also apply POR15 as a paint or spray

You are over thinking this.
Get it all coated. If fits are too tight sand/cut/grind it back.

I'm thinking powdercoating..

Originally from the factory the rear subframe was hot-dip galvanised.
 

You have two different goals here...

If you want to refresh and protect the subframe then the easiest is to bring it to a company that does hot-dip zinc plating. They usually have acid baths to take off any old zinc from material and will then re-dip it for you. Usually guys like these work in hundreds and thousands of kilograms of steel to strip and plate (eg. old fences, industrial carts, etc. etc.) and a tiny subframe won't bother them

That should take care of protecting the subframe for the next 20 years or so. You will need to clean out some threaded holes, but the zinc layer thickness on hot-dip is much bigger than electro-plating, so it lasts a loooong time.

Not sure it will show up, but here's a stripped and re-dipped subframe from a toyota-Elise:

The structure was already designed for hot-dip (no enclosed voids) and an 'after' check on the dimensions on the subframe above showed no distortion compared to pre-dip.

(BTW.. Might also be a good idea to check the lower rear 'legs' of the subframe for crack development and if found have them ground out and welded to fix...)

Second goal is then preventing the galvanic corrosion and white powder developing between the steel subframe and the alu chassis once it's back together.

Solution here will be the proper application of Duralac on the mating surfaces and also using the excellent new coated and shims Gaz has been producing.

Once it's all bolted back together I guess you could douse the joint with a conserving wax type coating (eg. Tectyl, Waxoyl, etc.) to stop moisture creeping in, although the duralac should already basically seal the joint itself once it fully hardens once it's all assembled.

Bye, Arno.

 

I'm thinking powdercoating..

Originally from the factory the rear subframe was hot-dip galvanised.
 

You have two different goals here...

 

If you want to refresh and protect the subframe then the easiest is to bring it to a company that does hot-dip zinc plating. They usually have acid baths to take off any old zinc from material and will then re-dip it for you. Usually guys like these work in hundreds and thousands of kilograms of steel to strip and plate (eg. old fences, industrial carts, etc. etc.) and a tiny subframe won't bother them

 

That should take care of protecting the subframe for the next 20 years or so. You will need to clean out some threaded holes, but the zinc layer thickness on hot-dip is much bigger than electro-plating, so it lasts a loooong time.

 

Not sure it will show up, but here's a stripped and re-dipped subframe from a toyota-Elise:

 

 

The structure was already designed for hot-dip (no enclosed voids) and an 'after' check on the dimensions on the subframe above showed no distortion compared to pre-dip.

 

(BTW.. Might also be a good idea to check the lower rear 'legs' of the subframe for crack development and if found have them ground out and welded to fix...)

 

Second goal is then preventing the galvanic corrosion and white powder developing between the steel subframe and the alu chassis once it's back together.

 

Solution here will be the proper application of Duralac on the mating surfaces and also using the excellent new coated and shims Gaz has been producing.

 

Once it's all bolted back together I guess you could douse the joint with a conserving wax type coating (eg. Tectyl, Waxoyl, etc.) to stop moisture creeping in, although the duralac should already basically seal the joint itself once it fully hardens once it's all assembled.

 

Bye, Arno.

 

Just be careful although  hot dip galvanizing is the best option - There could be issues re-tapping the top chassis mount - As i explained in a previous post the thread on this is recessed and trying to get the tap to engage into the original thread could be difficult and it would be easy to cut a new thread . there may be ways to protect/mask but make sure you are aware of this before going down this route 

In the picture looks like they have left some bolts in the subframe to protect the threads? could you not do that to all places where bolts go and the use plenty of ACF when fitting new bolts.

Putting in bolts helps, but you do need to heat them up when removing as the zinc does quite a good job of working as a threadlock when it's solidified 

Not sure if there's anything else you can use like carbon threaded plugs or similar heat resistant materials that could be used to stop threaded holes from filling up and doesn't adhere to the zinc. (hmm.. does hot-dipping zinc even stick to stainless bolts? Could be an option if not...)

Localised heat will melt the zinc anyway, so threaded holes could be hit with a small acetylene or propane torch tip to blow the zinc out and then chase the thread to clean it out.

Bye, Arno.

Putting in bolts helps, but you do need to heat them up when removing as the zinc does quite a good job of working as a threadlock when it's solidified 

 

Not sure if there's anything else you can use like carbon threaded plugs or similar heat resistant materials that could be used to stop threaded holes from filling up and doesn't adhere to the zinc. (hmm.. does hot-dipping zinc even stick to stainless bolts? Could be an option if not...)

 

Localised heat will melt the zinc anyway, so threaded holes could be hit with a small acetylene or propane torch tip to blow the zinc out and then chase the thread to clean it out.

 

Bye, Arno.

Arno - Agree with all this - Not sure about zinc sticking to stainless or carbon inserts maybe someone may be able to advise - I Think bolts or something would definitely have to be used in the top mounts  as  the threads are recessed by a few cm and the problem (from memory) is that  the hole does not allow an m12 tap to move freely (in and out) so basically it bites (slightly) before it reaches the actual thread !!! - if you look at the inside of the hole you can see slight marks where the tap has just bit !! -It is this reason that makes it very difficult to get the tap to locate in the original thread and i would be concerned that cutting a new thread could result in a weaker final fixing - I  how this issue was dealt constructed  during original construction of the subframe -Does anyone know if it the thread is a nut welded to the inside if the boxsection as (similar to threads on different parts of the subframe) if so surely the problem would have arisen during manufacture !! - Im suspecting it is not a nut but just a thread cut into a certain thickness of material - Again maybe someone may be able to advise.

Cheers

Edited by Spitfire Engineering, 05 January 2019 - 04:01 PM.

Where from Gaz?

They're made by Spitfire...

Where from Gaz?

Sorry, that wasn't very clear  

The description of the zinc epoxy is from the manufacturer's website, we make the shims.

£15 each or £28 a pair plus del and vat.

OE, when you can get them, are £17.74 + del and vat.

The steel is 255, blasted and then coated as described above. I cannot think of a superior way to do these within the realms of reality or what customers are prepared to pay for them  

Whichever comes first!

don't powdercoat. As soon as you clamp the surfaces together the powercoating will crack and expose bare substrate. Use high zinc paint if you're going to refinish and overcoat with a two part epoxy coating that will chemically bond to the substrate

Right, think this is the route I'm going down with the epoxy coating you've suggested.

https://www.bilthamb...gs/epoxy-mastic

Obviously I'm cleaning up / degreasing the subframe as much as I can, but can I go straight on with the zinc paint or would etch primer be better before, or would it need sandblasting etc..?