Those triangular panels are a weakness. Mine looks like double skinned, and from day 1 I dribbled ACF50 into a single small hole which us obvious on the top. This drained in overnight and did that for a few weeks until it no longer drained away. I also covered the chassis with another layer of wax protection. Mine was done with golden wax at the factory by request. The other weak point is the double skinned crossmember ahead of the rear wheels, and gravel rash will compromise the finish and it will start to corrode.
The post 15/16 chassis needs attention and most cars won't get enough on these points. I saw one at Brooklands, low miler, not old, and the triangle panel was completely surface rusted. Don't be complacent with the galvanised chassis either. The surface can create a white powdery residue, so it needs watching.
