As the XJS and other Jags from that era age, there is no escape from the eventual vanishing beauty of the finish. Here are a few different stages of this dilemma.
1) White spots in the clear coat: This condition occurs when elements have affected the underside of the clear coat. Sometimes this can happen from a defective respray. Anything from a tiny piece of trash or oil from a simple fingerprint on the paint before the clear coat is applied. Or this can come from road debris hitting the paint. For this reason, this normally shows its ugly face on the hood/Bonnet or roof if a hardtop. This is the least costly to correct as the paint is still in good shape and the clear coat can be sanded, sealed, and resprayed.
2) Open spots in the clear coat: This condition is where the clear coat in the first description is not addressed in time then it begins to peel and the paint is now exposed. As described in number one, the solution is to sand down to a safe level as even as possible then a sealer is used to seal the paint to clear coat transition then clear coat as a final finish.
3) Paint damaged: This condition is where the paint has been exposed to the elements for too long and the paint is faded so badly that it is dry and dusty to the point that rubbing with compound wipes the paint completely off right down to the white/light gray primer. The solution to this condition is a complete respray, of at least the damaged panel. The parts that suffer from this condition are the hood/bonnet, top, and trunk/boot on a Sedan or Coupe. The same panels are affected in this condition on a Convertible minus the top.
4) Color matching: If the car is in the condition described in 3 and the side panels, for the most part, are in great solid shape, then the top affected panels can be repaired alone and where only part of the side is being repaired let's say the front of the fenders from rock chips same as the hood/bonnet. This will need to be blended. To do this correctly, one cannot use the paint code on the driver’s door sticker for the ordering of paint.
Remember the paint is being blended and all paint changes pigmentation as it ages. Therefore, a good auto body supply will have a special camera that is used to shoot the current pigmentation then insert it into a computer, which will give the correct mixture to color match the car as it is in real-time. If the paint code alone is used as a guide, then you will have a difference in the shade from the repaired to the un-repaired. You have seen this on cars and now you know why and it is so unattractive that the excitement you had for the car like a puppy humping his owner's leg, is suddenly diminished like a tall beautiful flower that someone just poisoned.