I had all my paint(urethane) mixed at one time and am painting each panel as I go. As long as the paint was all from the same batch it should look O.K. painting panel by panel.I also am using POR-15 on the inner fender aprons & the backside of all my sheetmetal parts. That should hold the rust off for as long as I own it.
make sure your gun settings and air pressure are the same everytime you paint or it could slightly change the color. also make sure you keep the gun the same distance from the part. nevermind how I learned this