I am currently taking the paint off this alloy tailgate.
The inside so far has been done with a sander.....a disc type and I will finish off that side with a small sand blaster I have that recycles the medium.
The corrugated side is being done strictly with the needle gun.
Metal thickness on the corrugated side is about a 1mm thick so I have to be very careful.

DSCN0644 (1) by john smith, on Flickr

DSCN0656 by john smith, on Flickr
DSCN0657 by john smith, on Flickr
DSCN0659 by john smith, on Flickr