Othersider,
I can only tell you what works for me...there are almost as many ways to paint as there are rocket models these days. Here are a few tips that I use:
NEVER brush on paint, unless you are fine painting a micro-rocket or a very small part. Spray paint is the prefered method and has probably painted the majority of rockets that have been built. Some people use an Airbrush to paint their model; this is fine if you have an airbrush and are painting smaller rockets, but doesn't work as well on larger models. Some of the largest models are actually shot with a "big gun"; I know of several people that have taken their models to an Automobile painter for the finishing coats, but these were rather large models.
Sand your model first. If you have spirals, fill them if you wish, but lightly sand the surface with some 220-400 grit paper. Roughening the surface helps the primer to adhere. Sand the fins, body tube and nosecone.
ALWAYS use primer! It gives you a solid base coat and primer is made specifically to adhere to different materials. It is the interface between your model and the paint. Paint alone can have difficulty adhering to some raw body tubes, nosecones and fins, where as primer is made just for this purpose. There are different kinds of primer put there and I prefer the Krylon white for regular priming and the Krylon Gray primer more for filling as it is thicker than the white.
Primer also is used as a filler and surface smoothing agent. That is why most of them are sandable, although some sand easier than others. I usually apply 3 or four light coats, then wetsand under a sink with the water running and I use Wet/Dry sandpaper during this process. If you sand down an area too much, let it dry and prime again. Primer is absolutely KEY to a good finish!!! When I am done, up to 10 coats of primer have been added/sanded/added/etc... you get the idea. Sanding usually starts with 220 grain Wet/Dry sandpaper and I finish with 400-600 grit sandpaper. The surface will feel like glass...that's when you know it is ready.
Filling...during the primer process, you can fill any divots or imperfections after the first coats. I have used Elmers Filler, Body filler and others, but my favorite is Evercoat Formula 27 General Purpose Filler, which can be purchased at most boating stores. It mixes easily, apply very smooth and creamy and dries like concrete. The beauty is it sands into a fine powder and doesn't clog the sandpaper like other fillers.
Now, you have filled, primed sanded, filled primed and sanded and the surface is ready for paint. Make sure the model is clean with no fingerprints, then hit it with a TAC rag to remove any lint. For paints, I use Krylon Spray Paints...they are inexpensive, dry extremely fast and can be recoated at anytime. Also, other paints like Testors can be painted over them without compatability problems. Whatever you use, stick with one paint for the model, so you keep problems out of your finish.
I lightly dust the first layer of paint, just enough to add color. Other coats are added, up to 3 or 4 coats total, until the surface is finished. A clearcoat is recommended to protect the paint job, but thats your call. Note too that I let the final primer sit for at least 48 hours before adding color. Any other colors can be added afer 48 hours if you use KRylon, otherwise wait nearly 4 days to do the next color as it really takes that long for some paints to harden.
Sure hoped this helped...and if it seems like I spent alot of time on the priming, you are right! Remember this...your finish is in the filling and primer...Paint is only used for color.
Godd luck,
Carl