The fashion now is to restore old trucks especially, but some cars, as well, to good running condition but leave the exterior paint and rust alone for a "vintage" look. These usually are sprayed with a few coats of clear-coat to arrest any further rusting or paint loss. I wasn't aware this was a thing until I saw it done on one of those car restoration "reality" shows. I have to admit I like the look - and it makes for a lot of great subjects to paint. My only problem is that the clear-coat makes the rust shine, which looks odd because rust doesn't shine. At least, it doesn't have to shine on my paper. This one worked out nicely, though a little darker than I meant to go with it.