So Dan, the issue you are having is called paint fry, and there are a few reasons why this could have happened.
1. You got a bad batch of paint, yeah sometimes that happens.
2. The undercoat was still de-gassing when you applied the top coat and caused the reaction to occur. Even if you leave it a few days this can still occur with some paints and the de-gassing process changes time with ambient temperature.
3. it is simply a reaction between the top coat and undercoat or undercoat and primer when the top coat was applied.
Unfortunately the only way to fully fix it is to strip the helmet back and re start the process.
you could try and sand it back lightly then hit it with the old wet n dry and try and reapply the top coat in the hopes it doesn't Fry again.
Its just a minor set back so push on as this will be a spectacular helmet once completed.