Lay the carcass on its back side so the door allows some extra light to enter from above. (I propped the whole thing up on blocks) I use a disposable chip brush for priming. Sometimes cheap chip brushes drop bristles but it's easy enough to pick them out now, and it is just a prime coat. Prime the back side and top as far as you can comfortably see/reach, then flip it over to prime the opposite side. A work light may prove useful since the door no longer lets in light. Once this is dry enough to sand, usually a few hours, I sand it lightly to remove any ridges and flatten any drips I missed, then paint the interior with a good grade of enamel paint. Rustoleum gloss white oil based enamel was my choice. This will have to dry overnight.
Now it's time to prime the exterior. For the top, you won't have to prime under the aluminum flashing, just the vertical inside edges of the sides with a bit of overlap onto the top along the edge, but you do need to prime the bottom below the flashing. It's not a bad idea to run a thin bead of calk along this joint and wipe it down with a putty knife to fill any unsightly gaps before priming. You don't want to wipe this into a fillet with your finger or you may run into trouble when the aluminum has to fit (the last thing, to keep it safer from scratches and dings).
Take care around the fenders to get a nice even layer of primer. After this prime coat is dry, touch up with light sandpaper. Layout the paint scheme and mask off, then paint the bottom (again taking even more care with the fenders, especially if it's a dark color) When you get to the door, paint around to the inside edge, where the ⅛” round-over ends, you should be able to do this neatly without masking it off. Remember when running masking tape around the side edges to the top to keep them lined up with the original line and don't let the tape lie flat and wander off around the curve of the edge. This is an important detail to remember (to have remembered) back when you were marking the line for the flashing along the top. It's rather like marking a waterline on a boat - it stays the same height all the way across the sides, around the edges, and across the front and back. Remove the tape as soon as you are satisfied with the finish coat (another boat-oriented reminder is to always work with a wet edge when painting a large area for a smooth coat). If you leave the tape on while the paint dries it will leave an edge and may even pull some paint with it when removed later. By removing it now, when the paint is starting to set but not yet dry, you will allow it to settle and taper to that edge. Let this dry overnight.
Next day, mask and paint the top, remembering the portion under the flashing won't need this coat either. Remove the tape when satisfied and let dry overnight. Install the axle and wheels, remembering to get an accurate length for the axle which includes enough for the end caps. Don't make it too tight a fit. Fashion the bottom by cutting it to width so it will slide, remembering to allow for several coats of primer/paint, and be sure to leave enough for the tongue. Slide into position, mark across where the top lines up, also measure the distance from surface to tongue when the bottom is level and make a note of this measurement for the little front support that will be added later. Remove the bottom, lay out the tongue (again, I made a pattern first out of ¼” ply to mark the tongue on the bottom and then to be a guide for the router). Make a note of the measurement from the back edge of the bottom to the pencil line, prime the bottom all over. You can prime one side at a time and let dry between, or hang it and prime both sides at once.