Prior to applying the underbody coating I 3D printed a heap of plugs in various sizes and inserted them into any nuts/rivnuts to stop the underbody coating from getting into the nut threads.
All other openings in the underbody and engine bay were simply taped over with masking tape.
The rest of the shell was also masked off with paper & plastic sheet. Note the two mating surfaces for the steering rack was masked off too.
Underbody coating is a messy job so the floor was covered in old bed sheets and I wore a full body coverall, nitrile gloves and safety glasses. It was applied to floorpan & inner wheel guards & boot floor with a schutz spray gun.
After the underbody coating was completed, everything was painted with 2 coats of satin black.
At the same time I painted fuel line/battery line underbody covers with the coating.
The middle area of the lower firewall had been masked off as it will be painted in the body colour. The black underbody coating seen here on the firewall will be hidden by the front subframe once fitted.
Inside the rear valance corner closing panels was also sprayed with a heavy coating of cavity wax (Valvoline Tectyl 506), using a tube nozzle poked through the small openings in the rear panel :
The excess was left to drain out while it dried :
I also took the opportunity to check the fuel line/brake line/battery cable clamps that bolt to the underside were still okay. It's lucky I did this as while I was fitting the fuel lines I remembered that I had previously messed up and ended up with a ~12mm gap between the lines & floorpan, which was unacceptable as it fouled with the steering rack gaiter :
I had to completely remake all three fuel lines (5/16" feed & return, 1/4" vent) - single piece lines that route from the rear to the front of the Mini.
The clamps (for fuel lines, battery cables and rear brake line) were originally hand-made however since purchasing a 3D printer, I designed new ones in CAD. After prototyping multiple versions to get all fitments just right, these are the final designs however the final prints will be black coloured PETG.
Boot floor fuel line clamps (the open line shown here will be connected to the fuel filter output) :
I also 3D printed some spacers for the underbody covers as the custom clamps are taller than the factory fuel lines.
Before starting the high build primer, I decided to make a dolly so the shell could be moved around the workshop and have unimpeded access to the panels to be painted. It will also be used for trailer transport to the paint shop. It was a simple and sturdy design made primarily from 76x38mm patio tube with 6" castor wheels. This photo is the frame clamped together for storage.
The round tube bolts to the front steering rack mount and the upright pieces bolt to the rear shock absorber mounts, with a ~2m piece of patio tube between. Only afterward did I realise it probably would have been easier to bolt the rear part of the dolly to the heelboard instead of the rear shock mounts but I wasn't going to change the design.
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