I would power wash the surface before doing anything.
If the old paint is still holding up, it can serve as a base coat. If you are going to hand letter it, coat it out white first. Then use pounce patterns and cut in the black. It's much easier than trying paint white letters over a black background. You avoid double-coating the white letters this way.
I would use Ronan Fast Dry Background enamel, assuming it's still manufactured. By the time you finish rolling, it may be dry enough to start cutting in the black at the place where you started. I would use black lettering enamel.
The only problem with Ronan Fast Dry is if you wait too long. It gets too hard. So if you don't cut in the black the same day, do it the next.
I'm sure many would coat out black and lay white vinyl, which works fine, too.
One more method would be to use liquid maskant, like Spraylat. Coat out the panels first with one color (probably the white). When dry, roll on the mask with a thick nap roller. Do one side of the canopy, then, when it's dry to the touch, roll a second coat on the same side. You want the mask two coats thick and you don't want to wait till the next day to apply the second coat. After you've finished masking all sides, use a pounce pattern and cut and peel the mask for the second color.
The Spraylat mask will have a heavy orange peel texture when you roll it on, but that doesn't matter. It cuts beautifully with hardly any pressure. You will use at least a five gallon bucket of Spraylat for this job.
This method lends itself to spray or roller for the color coats. If I were going to use automotive paints or polyurethane paints for a long lasting job, I would probably also go the extra step of scuffing the surface before laying the first color. A maroon Scotch Brite pad on a DA sander works well. Then tack off and apply the first color.
If you've never used Spraylat masking before, it's important that there not be any thin areas—it may not come off easily otherwise. At the perimeter of the area to be masked, lay a tape border, either 2-inch masking or 2-inch low tack (whatever sticks but doesn't pull the base color off). Then roll the mask right up onto the taped border, but not beyond. This makes it easy to remove the mask when the paint is dry.
If you pull up a wrong piece of mask accidentally while you're cutting the letters, it won't stick back down. Any wrong cuts in the mask need to be patched with a piece of masking tape.
I have lettered dozens of aluminum-skinned semi trailers this way with automotive enamels.
FYI: it takes 7 gallons of paint to letter an old style Mayflower trailer.
Brad in Kansas City
