The firewall is one of the earliest pieces that can be worked on. Materials to begin with are easy to source. It is made of a 15-18 mm plywood frame, PVC foam (usually 70 kg/m3 but a wide variety is approved). The frame is then skinned with 2.0 mm Okoume plywood and the forward side is made fire/heat resistant.
Dimensions
Important! The width of the firewall is not cut to final size in the beginning. The frame, with PVC inserts and plywood skins is assembled with 5 mm excess width on both(!) sides. That is, to 850 mm total width.
The reason is that the 840 mm width has to be achieved at the forward side of the firewall! However, the longerons of the fuselage join the firewall at an angle which requires the firewall to be "oversized" at the rear end. The final width of 840 mm is later achieved by sanding the firewall sides at the angle of incident of the longerons. This will be slightly different on each build since the exact angle depends on the exact properties of each ones Oregon pine strips used for the longerons.
Frame
I chose 18 mm Okoume for the firewall frame, but planed the thickness down to 16.5 mm to save some weight. That was not done right away, but later when realising that my 18 mm plywood density is on the higher end compared to other builders.
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PVC Inserts
The cutouts to save weight are filled with PVC foam.
The cutouts were glued into the frame using micro slurry. Once cured, the PVC foam was first cut to thickness using a Japanese hand saw and then sanded flush to the frame.
Before skinning the assembly, the PVC foam was sealed using dry micro. Once more, a sanding excercise followed once the micro was cured.
TODO: Photo!
Sanding
Once the PVC inserts were fitted. They had to be planed down to the correct thickness, flush with the frame. The first rough cut was done again with a Japanese hand saw which works just extraordinary well for so many things.
Then, I prepared 1m long beams of XPS foam in 60mm thickness where I glued sanding paper on. Not directly, but on one layer of 200g glass that I layed up with excess resin from other work I was doing.
TODO: picture of sanding blocks
Sealing and Sanding (again)
Once the firewall was flush.