What you have on that spoiler is actually millions of fractures in the clear coating. Whether it be a gel coat or a normal clear coat paint they are fractures or cracks that are clean through through the coating and that's why they can't be sanded off. They are not in the base coat. Maybe there is some technical name or some coined name for this phenomenon but is is simply a failure (cracking) in the clear coating so I call it clear coat failure.
I have seen numerous examples of this type of failure and am not really sure what causes it in all cases but can tell you that when a car's clear coat has been reduced to a state where it is too thin from over polishing this cracking will occur and will accelerate getting worse as time goes by. This is the particular case with my Buick which has steel panels. When I first started detailing cars I practiced on my Buick with my first polisher, (Cyclo) at the time I had yet to learn the rule of thumb "least aggressive/abrasive method first" and used lots of different over the counter aggressive products such as 3M Heavy Duty Rubbing Compound with cutting pads on my Cyclo and then followed that with finishing pads and a finishing polish creating the perfect finish.
A year or so down the road (doing this work for money) I bought a Makita rotary polisher and again practiced with it using my Buick and other beater cars, further reducing the thickness of the clear on my Buick. This was not me just blindly cutting away at my clear coat as I had done enough research to know better but rather a conscious decision to sacrifice the paint on my own car instead of making these mistakes on a customer's car. The result was the same exact cracking as seen in the photo of your spoiler, though likely with a different root cause of the failure.
So clear that is too thin and has lost it's protective properties against the sun's damaging ultra violet rays is just one cause of this phenomenon. Incidentally, I have purposely cut clean through the clear on the trunk lid of my Buick just to see if these fractures go beyond the clear paint and into the base or color coat. They do not go that far. Once the clear was removed from a section of my trunk lid, the appearance of the cracking was also removed.
I have also seen many examples of this cracking or fractured clear coat failure on Mercedes-Benz vehicles. Most of which have been one owner cars and have never been buffed out by detailers except for the dealer prep when they were brand new. The cause of this failure on these cars is unknown to me but I have quizzed the owners about how they cared for the paint and most say that they were told by the dealership that the car didn't need to be waxed because "It has a clear coat" :doh: so it seems in these cases the paint was simply neglected as a result of misinformation. :nomore:
Then there has been the Mercedes-Benz owner that claims to have waxed or sealed the vehicle thinking that over the counter waxes and sealants are akin to the force field on the Starship Enterprise. "Well, I wax it every other year" is something I hear very often and they are shocked when I tell them that OTC waxes and sealants last only a few months at best.
I had once detailed a GMC Safari that was a beautiful dark green in color and the passenger side door only, had this type of clear coat failure. When I sprayed it with a quick detailer and wiped it back off the door looked perfect but then in about 20 minutes when to QD dried from the cracks or fractures, the door looked the same. Why just the door?
I have seen this phenomenon almost in every case where I show up to detail a car and find it parked under pine trees completely covered in black tree dirt, sap and pine needles, suggesting that acidic erosion of the paint is also a common cause of this type of failure.
Though this phenomenon or failure is something that I see quite often as a professional auto detailer, it's cause is still very much a mystery to me except in the case of my own car and the cars parked under the pine trees. I can only assume that in "most cases" it is as a result of neglect in not keeping the paint protected from the elements.
In your particular case, my theory is that you have a spoiler that is subject to expansion and contraction due to temperature changes and has been sprayed with a clear coat with no "flex agent" mixed into the paint or has a gel coat as has been suggested by others and as a result of the constant expanding and contracting of the plastic or fiberglass material, the clear coating whether it be a gel coat of just a common clear paint with no flexing abilities, the clear has cracked.
These are my experiences and opinions. Take it for what it's worth. Food for thought that I hope helps or enlightens someone or inspires someone to research the subject further. TD
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