Why box store spray cans are not meant for models.

I have never in all my decades using store bought spray paint directly from the can, rather then how you did it. Had a problem. Now, having been brought up on a farm and owning a farm and restoring equipment including IH and JD tractors and repainting, the paint is thicker and dries faster. I see you bought a paint for Farm equipment. Its made to be thicker compared to paint like RustOleum. I could actually see the difference when I ran out of IH Red but had a can of Gloss Red RustOleum and when dry saw the depression in the paint where the IH was thicker (this was for bottom of fuel tank were the difference would not matter, I thought. Ended up redoing the whole thing). I see were some will spray paint into a cup and then take a paint brush to try to touch up a scratch, etc. When you do that, the thinner evaporates quite quickly which needs to be replaced when doing touch up. I did the same thing when restoring Corvettes and needed a slight touch up, add some thinner and then brush.

But again, have done engines. tank cars, passenger cars, and structures without a problem. Different manufacturers of boxcars have even different shades of Boxcar Red coloring.

Just my 2 cents.

KRYLON is the answer that you seek. Easy to apply, even coating,
thin coats, dries quickly, compatible with almost all materials
and, last but not least, colors usually are a dead-match for the
original train colors.

I used primer also.  It's a double color car.  Primer, then yellow.  Mask the yellow, then green.

I'll stick with model paints.  I've learned my lesson.

It's a lot of work to re-do a train car.  Strip paint, remove detail parts, prime, paint, tape, paint, clearcoat, decals, final clear, re-assemble.

Messing up on one step and having to start all over is very disappointing.

I have done great with just normal gray primer from the local box store. 
here is one I'm still working on needs another light sanding with 800 - 1200 sand paper and another spraying as there is areas not shown I did not get correctly. 



I did this with a spray can primer from Wal-Mart and my airbrush which I will never do again, ( the airbrush) takes to long to get a covering. I will use a spray gun or spray can ( most likely spray can Krylon most likely) I have never used Autobody primer, not needed. 

Auto primer is thicker. Made to hide the imperfections caused by sanding. Found that out when restoring Corvettes and using a regular primers then topcoat. I could see sanding marks in sunlight. Had to re-do the removable hardtop and back down to bare fiberglass and start over. Came out perfect paint job and a 1st place trophy.

I have always made sure I used same type of paint, not necessary same brand. I have had luck doing that. I either use all acrylic or all enamel.

My orange passenger car, I used enamel primer grey then a mixture of enamel Orange with a little enamel white to tone down the color a touch. 

By the way the primer was store brand enamel grey

Tru Color.   They make the best railroad paints.  I should have just found a green that was close enough in Tru Color.