I did all my fuse interior painting prior to riveting; I didn't want to be blowing paint in an assembled fuse, with all the other expensive bits around.
I used S.W. JetFlex WR (water based polyurethane) The stuff is as thick as molasses in the winter (even when thinned 20%), but goes on very easily with a 1.8 mm tip @ 20 PSI at the gun. I really like this stuff; super tough. I used Stewart Systems water based primer under, and am very happy with the results.
Tho, if you want to go this route, buy a gallon in your color of choice. I went by quarts and found one batch to be a noticeably different tone than the others ... girrrr. I probably used 2.5 qts total (about 1/4 of that was left in the gun/hopper on cleanup because the stuff was so thick)