Yellow is a lousy primer

At least in model paint form it is.  Yellow will not cover black worth a darn.  For a primer under green I suggest light grey or silver.  Tamiya makes a nice primer in a spray can if you don't have an airbrush.

Then if you feel the green will look better if sprayed over yellow, you'll be able to spray a light coat of yellow over the light primer.  That way you won't hide all the details.

Scott Chatfield

That's what I was thinking.

That's what I was thinking. Luckily I had an Amazon gift card around, so some primer is now on the way.

Toothbrush

I know some guys might not like this one, but a medium toothbrush can usually remove the paint pretty well. I also prefer to put in a gray auto primer before airbrushing.

toothbrush - electrify!

I've been using a cheap electric toothbrush (old Spongebob and Dora the Explorer electric toothbrushes) and they work pretty nicely.  I've used a couple different solutions on different paints.  91% isopropyl alcohol; if that doesn't work, I've used LA's Totally Awesome, and if that doesn't work, I've used purple Super Clean.  For the really tough stuff, I've added slightly abrasive whitening toothpaste.  

Gets the stuff off pretty well for most stuff.  For example, this is an Atlas F9 that was in silver ATSF warbonnet colors.  The yellow was pretty much impossible to remove, but took a gray primer nicely.  It's currently being repainted in E-L gray-maroon-yellow.

Janet N. 

A toothbrush is definitely

A toothbrush is definitely the tool for the job. If IPA doesn't work, I often find that Superclean works on the paint (https://superclean.com/). Neither Superclean nor IPA are super friendly to your skin, so wear nitrile gloves while you're handling the shell. What I've done is use two chambers for stripping shells my father made. He used a length of 3" PVC pipe that's about the same height as a 5 gallon bucket, and he added a cap on the bottom end, and a screw fitting with a screw cap on the top. I fill the pipe about 3/4 with the stripper, and then tie a loop of fishing line on the shell that's long enough to suspend the shell in the stripper with the knotted end of the fishing line kept above the end of the screw cap (usually 4 lbs test). I drop the shell in the stripper, and holding the knot above top of the pipe I screw on the cap.

I keep the pipes in a 5 gallon bucket in order to keep them from tipping over. The nice thing about this system is the pipes are completely sealed, so you shouldn't lose your stripper to evaporation, and you don't need to breath a lot of it while you're removing the shell to clean off the remaining paint.

I've found that paints differ quite a bit in how they react to IPA and Superclean. Some dissolve readily while in the stripper, some fall off the moment you rinse the model off, and some require quite a bit of scrubbing with a toothbrush and multiple soakings (especially several Bowser production runs I've dealt with). Some, like Rivarossi passenger cars, require a soak in IPA followed by a soak in Superclean, and the paint just comes right off.

As for primers, don't forget that Tamiya makes a white primer, too. It's great for light colors, like caboose red and yellows. It's amazing how much of a difference the primer color can make to reds and yellows.

In the old days ...

We used brake fluid - does that no longer work?

The problem with brake fluid

The problem with brake fluid is it attacks a lot of the modern plastics and makes them brittle. If you're stripping brass, it's fine, but if you're dealing with a plastic shell use something kinder like IPA or Superclean.

Pink primer for yellow

A while back I asked a group of airbrush artists what they recommended for a yellow paint primer. They all said either yellow or pink primer. What I do now is prime in gray, then mix 3 parts red, 1 part white and cover the gray. Then yellow goes on top of that. I personally think it makes the yellow pop better.

Craig

air eraser

If you have an airbrush, probably the best alternative is an "air eraser" - which looks remarkably like an airbrush but shoots mildly abrasive grit. I use aluminum oxide but some have reported good results with baking soda. I've done the toothbrush thing, but that's a lot of work, and the toothbrush has a hard time getting into lots of the crevices. It's fine for getting the majority off, especially for some model paints that come off easily. But if the paint is in, for example, stair wells, in some of the smaller details such as under brake wheels, and especially under grills or mesh, there's just no beating the miniature sand blaster called an air eraser. I've used it on brass models and also on plastic - it works very well. A quite reasonable air eraser can be had on Amazon for about $40. I use the same compressor that supplies my airbrush.

I use Vallejo paints, and they have a line of primers. I usually use grey, but occasionally I'll get some that is closer to a base coat that will eventually be on the model if that color is problematic in one way or another.

That sounds useful

Also, do you recommend a certain compressor? I have some "canned air" right now, but I think I ought to have a compressor for the future, and I don't know which one to get.

Compressors

I mostly power the air eraser with the little compressor that I use for the airbrush. Mine is an Iwata SprintJet, but lots of others would work. It does mean that the tip of the eraser has to be a quarter of an inch from the model, but that's not a major issue. I do have a much bigger Dewalt pancake compressor that's intended for other tools, but with a $5 adapter it also can power the eraser. It definitely makes the eraser quicker, but it isn't a game-changer. If you have a big compressor, that's nice, but if you only have a little one, it's still way better than struggling with an old-fashioned toothbrush!