Doesn't it build up if you keep applying it?
Adams claims their advanced spray coating is good for 18 months, I re-apply every six months. You can build a few layers when applying. I always add two twice a year. In between I use a top coat or drying aid.
When I re-apply I wash with strip wash which only removes oil and wax’s and non ceramic coatings. I they apply the prep solution, 30% alcohol is all it is. I then apply a coat, buff it good and let it sit for four hours and then apply another coat of ceramic. Easy and cost effective way. 100 bucks, maybe
Let’s be honest, a professionally applied coat will last longer. Speaking from experience, it not worth it. Plus, read the warranty, most will need inspected annually at a few hundred bucks, you have to use a top coat every wash, sometimes only their brand. If you have a small rock chip, you are required to have it repaired and the repair re-coated to maintain the warranty.