The "firewall"

[Onbeachw320]

  I bought one of those "multipurpose tools" at Harbor Freight yesterday.  I am using it to remove the poor glass work on the fiberboard panels in the rear of the cabin of my new bodyshell.  So far I am using it to break through the areas that are clearly just fabric (no resin at all) and then I grab the area with a pair of pliers and pull.
  What I have found is that there might be a bit of black dope like sealing material between the fiberboard and the fiberglass, then the previous owner "glassed over" that area with a layer of woven fiberglass.  I am only dealing with the "inside" side for now, but will also address the engine compartment side as there is poor work there as well.  This is where I am going to get my first practice dealing with fiberglass work.
  I only started today, but tomorrow I am going to look at the underside and double check that the fiberboard panels are aligned correctly.  If they are not, I am going to need to free up the fiberboard so I can get a better seal before I reglass the panels in place.
  I know that I read in the past that these panels sometimes need to be replaced due to water incursion and that the fiberboards break down when they get soaked, so I will want to be sure that they are well sealed, which is why I started this project.
  I haven't heard back from the Lotus Supply folks yet (re: new bodyshell prep,), but figure I must be on the right track so far.  Any critiques on my angle here would be appreciated!  Since I don't have another example to scrutinize, I am wondering if ALL of the fiberboard is to be glassed over, or only the areas adjacent to the bond line.

[4129R]

Banks told me he used plywood and fibreglass for the inside to repair damaged fibreboard.

[Richard48Y]

This reminds me to search for thin aluminum honeycomb sandwich material.
Seems I have seen it available somewhere.

EDIT: Easy enough to find, but expensive in large enough size. 
I want a non-porous fire resistant material.

[Onbeachw320]

Richard48Y  ---  you are right.  It seems like I remember this from years ago.  I am thinking that the general aviation airplanes Beech Musketeer and (AM General) Yankee - Jim Bede design - had honeycomb wing coverings.  If I could find it, it sure would be ideal.  So far, I am about 1/2 way through tearing the bad material on the driver's side.  Itchy right now...

[Richard48Y]

So far McMaster-Carr has the best price but not certain they sell large enough pieces.
4' x 8' is available but $$$.
There is a manufacturer in the Denver CO area so shipping should not be too bad.
Need to measure mine, or maybe you already know since you are doing one?

[dakazman]

  This might be easier and lighter to work into the area . It doesn’t abourb moisture and can be bonded in  .  It’s a set of 3 panels but others are available.

    https://www.fibreglast.com/product/Polyisocyanurate_Foam_Sheets_440#pricing

   We also had a good, long discussion about firewalls a little while ago here . I made mine with three removable panels for access later on .

  Dakazman

[Exlimey]

AIREX foam core,used in boatbuilding may also be suitable and easily obtainable.

[Kendo]

Does the body flex enough that the new firewall should be glassed in with the body sitting on the frame? Or is it stiff enough to do free standing? I replaced mine in a nearly completely assembled car, so I don’t know.

[dakazman]

Does the body flex enough that the new firewall should be glassed in with the body sitting on the frame? Or is it stiff enough to do free standing? I replaced mine in a nearly completely assembled car, so I don’t know.

    I did notice a small amount of sag below window, < 1/8 inch , I had a guide that I made before removing the firewall.  The window lip did also rotate a little when the window was installed.
 Dakazman

[Tom999w]

I looked at my firewall the other day and it fell to pieces. Does anyone have a dimension template that's already made so I don't have to reinvent the wheel and re-do what hundreds of people have already done?
Also, does anyone know where I can get the OEM insulation material for the engine bay side of the wall? I'm going all OEM.

[Richard48Y]

I just rough measured mine at 38" x 48"
So a half sheet of whatever is chosen should be sufficient.
A pattern would be great if available.
I have a router, roto-zip, and various saws. 

[gideon]

Also, does anyone know where I can get the OEM insulation material for the engine bay side of the wall? I'm going all OEM.

This looks similar in some ways

https://www.woolies-trim.co.uk/product/1384/sound-deadening-felt

It has the black coating and fire resistance, so that fits.  If you order some, let us know how it compares.  For curiosities sake.

[GavinT]

Does the body flex enough that the new firewall should be glassed in with the body sitting on the frame? Or is it stiff enough to do free standing? I replaced mine in a nearly completely assembled car, so I don’t know.

I'm with Kendo on this one.
It's claimed that the firewall has structural properties but I don't imagine that amounts to much considering the properties of fiberboard.
That said, I'd wager it has sufficient stiffness to hold a set and potentially induce stress during marrying the body to chassis.
Much better to install the firewall with the body in situ and avoid any complication IMHO.

With my Type 54, the fiberboard used ~ 3-4 inch strips of chopped strand mat just around the perimeter of the panels.
I don't know if it was sealed across the middle parts. The fiberboard is a fairly compressible material and it'd only take a small knock to break the surface and allow water ingress to start the breakdown & disintegration.

I used 10mm PVC composite core material (pic below) for replacement panels - streets ahead of the cheaper Polyurethane foam stuff.