I have recently resprayed the front bonnet on my beetle with acrylic. I have done two guards previously and they turned out great but the bonnet this time has a fair few pin holes and air bubbles in it.

There was also a fair amount of orange peel and dry parts (it's starting to sound really bad isn't it ...)

The orange peel was not major, but a fair amount. The dry parts was from overspray.

I first done a light mist coat, then followed by a single coat (not wet, but adequate), followed by a double header coat, then lastly a single wet (with lots of thinner)

The wet coats was not overly heavy, so I don't think that was the issue.

I used a hvlp gravity fed gun this time (first time using it).
The gun had about 45 psi going in it. I have a mini moisture trap attached to the line also.

I sprayed in on a warm day (about 30 degrees). I added retarder thinner, but more of a generous splash.

Could someone point out what is the likely cause of the pin holes/air bubbles?

bubbles could be from moisture in the air line(water), pinholes could be solvent boil, if you lay too much paint on too quick this can happen, pinholes will bock back but moisture bubbles wont

I allowed a flash time of 5 minutes or so in between coats. The pinholes don't seem consistent, that is, there not over the entire panel. One section where I laid the paint heavy turned out just fine, if not better. So I guess moisture is the culprit. Thanks rexy.