I would use a stone chip over the primer or the PU if it is already applied. If the PU has been applied for a while I would give it a quick scuff with a scotch-brite before the stone chip is applied.
I used 3m stone chip over Jotan 87 and it worked well. I could have applied a colour coat but as the car is black I didnt bother.
You can apply stone chip with a shutz gun or a gravity spray gun with a 2.5 fluid tip, it might need a bit (about 10%) of thinner, I used 2K, if using the gravity gun. The stone chip will also give a bit of sound and vibration damping, but only if it is good and thick. Another tip is to use different colours for each coat so you can be sure you don't miss anything. Plan to end up with the colour you want and expect it to be really peely like the factory finish. I have tried Gravitex stone chip but I find it dries hard and although it's a good tough finish, it isn't proper stone chip. I have also used Wurth, it;s good but I can't get it for a reasonable price and the 3m is the best I have used to try and get a factory finish.
Ive attached a few photos (I hope) of what i used and how it turned out.
Factory stone-chip
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First coat over Jotun and seam sealer
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First, full coat of stone chip
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Final (I think 4th) coat
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I did not go for the 3M products as the ones I could find were not over-paintable, it seemed like the only choice for me was gravitex (or Raptor).
The original factory finish was ruberised and was over-painted though.
Jotun first.
Three applications of gravitex.
Then 2k solid.
I did not go for the 3M products as the ones I could find were not over-paintable, it seemed like the only choice for me was gravitex (or Raptor).
The original factory finish was ruberised and was over-painted though.
Jotun first.
Three applications of gravitex.
Then 2k solid.