I'm supplying a handful of A4 panels, I bought the Alu panels in traffic yellow so I can just apply black vinyl. Vinyl is going on fine but there are small areas, I guess they're bubbles where it isn't in contact with the board. it's only very small areas and can only really be seen at an angle but I'd expect this to be completely flat to the board?

I've ensured the surface is clean and now tried 2 types of vinyl just in case.

Is this to be expected or a I missing something?