The frame surround is some kind of anodised metal, and is prone to blistering, cracking and general corrosion. I don't think it's a problem, it just doesn't look pretty so I have had both doors stripped and refurbished by Pete at Langley Autocraft (who also did my windscreen surround a few months ago)
Blistered inside of door frame on passenger side
Paint peeling from top of passenger door
Extreme close up of drivers side door (hardly any corrosion on this side, but in for a penny etc etc.....)
The door is stripped, the frames removed, blasted then repainted and reassembled. As usual from Pete, a superb finish.