I spent 13 winters in Tampa putting air brushed murals on motorhomes and while the end result usually came out pretty well the preparation was something that caused near fatal heart attacks to the owners of half-million dollar motorhomes. The object was to basically destroy the area you planned to paint on but yet totally protect the surrounding area down to the slimmest fraction of an inch.
The system my business partner and I use, (used) to get the finest line possible was to 'BLUE TAPE', (plastic painter's tape), our paint line and then use pneumatic sanders to remove every trace of clear coat and raised paint lines from the coach. To accomplish it correctly it required actually sanding over the blue tape to achieve an absolutely straight line. In over two-thousand murals that we did the tape never failed.
I'm not sure how straight of a line you'll get by trying to sand up to a raised surface, I fear your might end up with a jagged and inconsistent line rather than a straight and uniform one. However, bear in mind that I've never prepped anything as small as a Reo so it might work... I'm just not sure.
As a means of shameless promotion I'll post a couple pictures of my coach so you can get an idea of what we did. All four sides of the coach are adorned with mural, (we used it as a display coach), and all gold colors are 24 karat gold leaf rather than paint. To
buy all the work would cost a bundle, (around ten grand), but when you do it yourself you just need time and a few hundred bucks in materials.
Oh, if I was working on a Reo I wouldn't try to do the whole job with one grit of paper but rather
three grits. Start the job with a moderately rough grit, (not aggressive), then on to a medium or moderately fine and finish with a very fine, something along the lines of a 1000 grit to remove all of the sanding marks. If you take your time it'll come out looking like something from Tiffany's.
A terrible thing happens if you don't advertise...
NOTHING!
Used to get invited to the Traveling Vietnam War Memorial to put the coach on display.
And Andy, (Colt45), keeps telling everyone my only talent is complaining. (Okay, it is my best talent.)
Good luck with your Reo.
Jack