I painted the car with Floquil's "Oxide Red", straight-up, no other colors. The first session went OK, but the next day I noticed that it just didn't look good. Several days later I had another painting session, and it dawned on me that styrene probably really needs to be primed. My first coat was just a primer coat! The second coat made the car look a lot better. The lesson learned here is that I need to apply a primer coat first. Note that I wrapped the couplers (which are permanently installed) to prevent them from getting too much of the over-spray. I had completely forgotten to fabricate and install the stirrups on bottom of the frame near the four corners, so that was done next and hand-painted.
For decals you have several choices.Oddball Decals (part #64-206) provides decals for H21, but not H21a. I made the mistake of thinking that when they mention "H21" that it would include decals for the sub-classes, but it does not; it is strictly H21 only. Also, I found their decals to be a bit small as compared to the C-D-S set.
The set I wound up using on this car was C-D-S Lettering set #S-282(a), which I bought from TMR Distributing (they are flagged as H25, but they also include H21a decals). It is a dry-transfer set, which I really don't like, but you might like them. Since C-D-S is no longer producing decal sets, these are getting harder to find anyway. For my PRR cabin car project I had great success with Greg Komar's "wet and slimy" decals (Greg no longer produces decals). His PRR part #390 should work fine for this car, if you can find someone who has these in their inventory; the same goes for Jerry Glow decals).
After spraying the car with Testor's Gloss Cote and letting that dry for a few days, I applied the C-D-S Lettering decals. There are two reasons why I don't like dry transfers; they are hard to position, and they "flake" if not sufficiently pressed into place. The one thing I like about them is that you can immediately spray the Testor's Dull Cote over them; no need to wait for them to dry. I like "wet" decals because you can position them exactly where you want them, and you get what the decal actually has printed on it. The inconvenience of having to wait overnight to dry is minor in my book. The C-D-S decals applied in the photo below show several parts where the printing was missing, even before I started to handle them. Oh well, some extra weathering in those areas will make it look like the lettering has worn off due to age. After all, these cars were 30 years old in my modeled time period. As far as the placement of the decals is concerned, nearly every prototype photo I have seen of an H21a has different lettering in different locations. I just picked one prototype photo and tried to follow it given what was in the decal set.
After the decals were applied, I sprayed the entire car with Testor's Dull Cote and let that sit overnight. This is the final car with the trucks installed.