Unlike the Herald and Vitesse, the body is one complete welded unit. Also like the Herald/Vitesse it can suffer from rust in many places. One place that is often the first, especially if there has been water leakage, is the floor underneath the front seats. Floor panels (handed) are widely available and are identical for all Spitfires and GT6s. Mine were “Steelcraft” from Taylor Pressform in Leamington Spa, which are now distributed exclusively by British Motor Heritage Limited (see illustration below from the Moss website). BMH was established in 1975 to support owners and the marketplace by putting genuine components for classic British cars back into manufacture, using original tools wherever possible. I bought my floors through, and with the help of John Kipping in 1980/81 when he was still the Spare Parts Secretary of the TSSC.

Moss Europe
The new floors can be seen seam welded in place (below):

The second area most likely to suffer from rust is the sills (or rocker panels in the US). These are load bearing as the chassis does not have the outriggers of the Herald/Vitesse. Below is the state of mine in the area just in front of the rear wheel arch (in fact both the sill and rear wing were corroded)!

The sills are in 3 parts as in the table below (from BMH):

To do do the job properly all three panels should be changed

When removing these panels the strength of the body may be compromised, many use ‘door gap adjusters’ or ‘reinforcing bars’ attached to the door frame while the sills are removed. This specially designed tool is designed to help adjust or maintain your door gaps. These are widely available, the one below is from the TSSC Club shop.

Another area which suffers from corrosion is the clamshell front bonnet assembly. Mine was shot in 1980, so I bought a brand new Stanpart spare which had suffered a little damage (in fact 3 had been been put on top of each other!). Nowadays new bonnets are still available from BMH for the GT6 Mk3 and Spitfire Mk4/1500 and also T.D.Fitchett.  T.D. Fitchett are also advertising new bonnets for the GT6 Mk2. All are around £1,500. Repair panels are also available, for example front wings for the later GT6/Spitfire are available from BHM/Steelcraft. Fibreglass bonnets are also available, for example SC Parts sell one for the GT6 Mk2 less tubes etc. for around £700. My bonnet can be seen below, both before and after its second restoration!