Not a Model S spoiler, but I have a Model X which was delivered new in Jan 2019. The drivers mirror is showing primer because the top coat apparently was sprayed too thin. Put it off until now (9 months) but getting clear wrap and tried to get the mirror cap with the thin paint replaced under warranty. Cherry Hill service told me paint issues must be claimed within 30 days of delivery to be covered under warranty. Wish they told me that at delivery. Never had a $100k+ car with such a “thin” warranty!
Check your original warranty paperwork. See if the 30 day policy for paint issues is included in that original warranty. They do not get to change the policy as time goes on, they must honor the original warranty as presented at the time of sale. If there’s no 30 day limit on paint issues in your original warranty paperwork, it doesn’t exist. They can say what they want but the paperwork speaks for itself. Your position will be that the car wasn’t painted for a 30 day life span, it was painted with the idea that within the limits of normal wear and tear, the paint should last the life of the car. If there is a defect in the materials they used or the workmanship in applying those materials, it should be covered along with the rest of the car. They’ll attempt to stick to their guns but that’s where arbitration or the courts come in. The paint, all the paint including the clear coat was part of the car you bought. So unless the warranty specifically excludes it, it’s covered along with the rest of the car. You are not nor are you expected to be an expert in paint and or application. The fact you didn’t identify the defect until it peeled off doesn’t mean they get a free pass as far as the materials they chose or their application process.
Realistically, it won’t cost you much to have the spoiler sanded and resprayed with clear coat. So it is worth the effort to force them to honor their warranty? Probably not. Still if you choose to push the warranty issue an if there is no original 30 day exclusion for paint, then you should be on firm footing.