What causes this?

[DaveBro]

These were done over chrome that been grind down with a die grinder. What causes the bubbles?

[Harleydad]

I put all chrome pieces in the oven for a cure cycle to make sure the chrome will hold up. If it does and you're coating with a solid color just use a scotchbrite to scuff it up. If it's a candy/transparent clean well and coat. If the chrome doesn't hold up and bubbles it needs to be stripped.
As for your bubbles are you sure you stripped it all off? Is your air supply clean and dry? What type of metal are they? It looks like outgassing. Cast metals need to be outgassed to remove all junk trapped in the pores of the metal during the casting process.
Here's my process:
1. Clean and Degrease
2. Outgas in the oven (50 degrees or so above cure temp, try an hour)
3. Blast and blow off with air
4. Wipe down with the cleaner off your choice, (Acetone, pre paint cleaner etc. No petroleum based products) allow to dry or use clean dry air.
5. Go over them with a hand held propane torch to remove any rag lint, dust etc.
6. Coat and cure.

[baz]

from what it looks i would say that the chrome plating or some other plating under the chrome (nickel or copper) was still there, and bubbled when it was curing

[bzer1]

Unfortunately this is an example of a couple of pieces that should just not be powder coated. I've had a few that have come out just like that. At this point the only thing you can do is strip em down to bare metal and hope for the best. This happens alot on rolled then chromed sheet metal parts. It's most likely one of the under layers that was breaking down and it lifted when you cured it. If you strip it down you'll most likely find those spots will be down to the bare metal. If they're test parts...lesson learned, throw them away and try something else. If they're needed parts, strip em down and blast to bare metal and prebake the heck out of them before you attempt to recoat.