I paint Scalecoat II, without a primer, on both brass and plastic. Sometimes I DO use an undercoat, especially on my Armour Yellow cabooses because the plastic they're molded in is translucent yellow and I want the model to be opaque.
I clean my models in a three step process, including brass car sides. (1) I wash the model in a warm soapy wash (I use basic Dawn detergent) and I use a soft toothbrush to scrub it lightly, but thoroughly. Rinse thoroughly too. (2) From this point on, I don't touch the model with my fingers. I let it dry, or I assist drying it with a hair dryer set on medium heat. On brass engines or models, I'll blow the model dry with an air hose off of my compressor, but with filtration so no oil spots come out, since my compressor is not an oil-less design. (3) I put the model on its holder, which is usually either a large hemostat or a toilet paper roll paper core, and then, using a 1" wide soft artist's brush, I brush Bestine (Heptane) over it several times, hitting all the surfaces that will be painted. I do this in my spray booth with the fan on. If you don't have a spray booth, do this outside.
It doesn't take long for the Bestine to evaporate..like 5 minutes...then, I paint...usually using Scalecoat II with no primer.
I've never had a model's paint flake off using this method. I've been airbrushing commercially since the early 1970's, and unless you're using CO2, you must make sure your line is filtered, with oil traps if you're not using an oil-less compressor, AND an in-line moisture trap on your airbrush hose (always). I've used the Paasche in-line moisture trap since I started many decades ago.
Let your model sit between coats until you don't smell any "paint odor", which means the paint has cured. You can speed this process up on brass models by placing the model in a toasting oven, with an in-oven thermometer clearly visible. Preheat the oven to no less than 150 deg., and no more than 180 deg., and bake your brass model for approximately ten minutes. Do NOT leave it in any longer, as it is possible that the heat will burn the paint. I've only had that happen once, when I took a phone call, and forgot about the model, and left it in for at least half an hour.
Plastic models are different and I've had good luck baking them at temps no higher than 120 deg. However, you never know what type of plastic your models are made from, and some have very low sagging/melting temperatures, so bake at your own risk. Using a hair dryer will speed up the curing, but the "paint odor" needs to be gone before proceeding to the next step. Hair dryers work great on acrylic paint, but although they speed up the curing process with enamel based paints, you usually still have to let the model sit for at least several hours. Placing it outside on a nice Summer day (in the shade) will work too. Putting it inside a car on a Summer day works too, but I've heard horror stories of melting plastic doing the "car" method.
I always strip the masking away after the paint coat is "dry"...not cured...like within ten minutes after applying it...usually sooner. Stripping the masking works best if the paint is still flexible, but not wet.
This protocol works well for me, on plastic models, brass car side models and brass models.
