It all depends how you do it...
I scuff the tail light, apply 3 light coats of VHT, let it sit 10 min in between coats.
Let it sit for 20 minutes and apply 4 coats of automotive or krylon clear.
Let it all dry for 24 hours. Come back and wetsand with 2000 grit until its dull.
Then use polishing compound to bring it to a glass like shine..
I have done 6-8 vehicles and none have had issues...
Most only spray the vht on, and no clear. Which dull's fast and looks like crap..