Opinions Please On What's Happened Here

[cudaken]

When you have rust damage, there will be bubbles in the paint, if there are cracks, the filler is to thick. Filler can all so be primer as well! They can be hard to spot till fresh paint hits them!  Ask me how I know. 

Cuda Ken

[JMF]

Might be going to look at it again

[b5blue]

Take a camera with a flash so you can stick it in hard to see areas.

[tan top]

 to do it properly , what ever is under there  , it needs stripping to bare metal