FGARDEN,
A couple of thoughts:
- Without understanding exactly where is the corrosion on your car, it's not possible to advuse you if a fibreglass repair to your XJS is cause to fail an MoT. If you want to understand the MoT rules relating to vehicle corrosion and repairs, see here:
https://www.gov.uk/guidance/mot-inspect ... -corrosion
- IMHO, irrespective of where is the corrosion, you don't want to use fibreglass to repair an XJS. Get the job done properly with removal of all corrosion and a new panel, or properly-welded patch procedure undertaken.
- If the corrosion is at the bottom of the front wings, behind the wheel, there is a strong likelihood that the corrosion has commenced from corrosion at the forward end of the horizontal section of the sill / inner wing area. If this is the case, the front wing would need to be removed and further repairs undertaken.
Perhaps if you posted a couple of photos of the affected area, we may be able to better advise you.
Cheers
Paul