Tom, the S3's have either a soft rubber spolier (early S3's) or a firm rubber spoiler (later S3's) Either way, some "
Flexible Rubber Bumper" paint will work well for re-painting the spoiler. Auto supply stores sell those types of paint.
On our '84 I repainted the black parts of the bumpers with such a paint. And I used TurtleWax "
Wet 'n Black" (the clear liquid in a spray bottle - not the foamy product in a spray can) for the spoiler. It took a few applications - daily for a few days then weekly for a couple of weeks but it ended up looking quite well.