If you want Alaska Railroad engines someone will have to paint them. You may as well jump in.   Here are some proven tips that may save you some grief along the way.

Gather tools and resources needed in a well lighted, clean work area.  Sharp scissors, fresh razor blade, tweezers, Scotch Magic Mending tape, and a clean sheet of paper.  Photos of the prototype and a previous successful model if you have one are a must.

Start with the lightest color and let it dry overnight.  If you are doing the same paint job, mask several jobs in advance, but DO NOT leave masking on for more than a few hours.  Take time now to weather grills, shutters and dark recesses that will be in contrast with the light color.   Use a wash of alcohol and oily black, mostly alcohol and just touch a loaded brush into the recesses where it will flow down and deposit the dark pigments. This will dry quickly and it will be much easier to touch up these areas later with weathering.

To avoid tool marks in green paint, use paper strips with Magic Mending Tape applied.  Use the factory edge of the tape to delineate the paint edge with the paper extending into the masked area.  Let the paper hang over or around a corner where you can grab it later.  The second piece of tape can then be used to mask the other loose paper edge.   Any small pieces of tape needed to work around obstructions should always be overlapped onto the paper at some point.  Check to make sure all edges are sealed down.

Advantages of Magic Mending Tape:  You can see through it to position it properly.  You can see where it is not sealed as air under it will give it a foggy look, so burnish those edges  down tight.  With care it will stretch around and over obstacles. When attached to a paper strip it is dimensionally stable and will hold a strait line.  To do arcs or curves, cut your paper backing on a slightly smaller arc or curve and the tape can be applied and easily trimmed to a larger arc or curve extending past the paper.  If removed within a few minutes it will not leave residue or attack base paint.  It is thin so paint cannot build up against it.  If you use the paper strip, call it the zip strip, it is easy to remove.  With out the paper strip you will have a messsssss.

Check for sealed edges and then paint.  Help your masking job out and do not deliberately spray paint into a tape joint.  If you know of a potential leak point in the masking job just avoid it by over spraying away from the gap.  Try and keep the gun 90 degrees to the surface.   With PolyS paint, start removing the tape within 15 minutes of your last coat.  This is important because you want the "soft green" paint to blend into the base instead of the edge of your masking.  When the masking is removed you will find some over spray here and there, not to worry so did the big guys.  If it is bothersome, remask the area to a natural break point and paint again.   You can probably incorporate weathering into covering the boo boo.

That is another topic.   Try this masking system, you'll like it.

P.S.  Let's do a paint check to see which paint we used.