You may can try your stencil again but this time use masking tape instead of paper. Put the tape on wax paper and punch your holes. Peel off the tape and transfer it to your rocket.
--OR--
You may can use a sponge, either natural or manufactured. It would create a more random effect and wouldn't be all white dots. I have used to em to texture walls and ceilings but nothing small scale like a rocket. There are a few ways you can try it. But try it on a scrap piece of something first, primed cardboard is usually my test area, or a plastic bucket. I would suggest using cheap craft paints to play with and see if you can come up with a technique to give the best results.
Technique #1:
Paint the surface whatever color you will use for the background or sky. For this we will say blue. Squirt or spray a small amount of white onto a piece of wax paper, coffee can lid or whatever. Dob your sponge in it, then dob it onto another scrap of something - newspaper, cardboard, terry towel or the like - once or twice to get off excess, then dob it onto your blue background. You should have a speckled effect. If the speckles are too crowded, repeat the sponging but use blue rather than white. It will only cover up some of your white speckles. Try using a sponge with a different pore size if you go back with blue over your white. When you dob your sponge, don't press too hard and don't smear it around. Straight down and straight back up, turn your sponge to change pattern then dob again, straight down and straight back up... repeat. You can try "rolling" the sponge as you dob, but it may smear.
Technique #2:
Almost the same but you will be pulling paint off rather than putting it on. You will paint the part to have the snow white. Then spray it blue. You will have to work faster than with technique #1 so the blue won't dry on ya and you can't dob it off. Dob your clean, paint free sponge into the still wet blue. It should grab some and pull it off. If it pulls off too much, you can use technique #1 to cover some back up.
When all is said and done, the finished product will be textured. You may can sand it, but more than likely it will mess up the pattern. You will have to build up with your clearcoat before sanding. And it can be a fairly long process. Follow the instructions on your clearcoat and spray on a couple of coats. Let it dry (it will feel like and orange probably) for the recommended time and wet sand it. Make sure your sandpaper can be used wet. I would suggest a high grit sandpaper but I recently went through the process and resorted to medium grit sanding sponges. My final sanding was with 2000 grit. Wet sand it and you will see dull spots appear. That's the sand paper taking off the high spots. Don't over do it with the sanding. Wipe it off and dry it. (I use denatured alcohol to clean just about everything but make sure it won't strip your clearcoat) Spray it with clear coat again and let it dry. You should notice a difference, less orange peel like. Repeat until nice and smooth. Or until you are satisfied.
I recently used a similar technique using saran wrap.
Here is the build thread, the last few posts have pictures of the finished rocket. (TRF still not letting my upload pics and the pics from photobucket will be too big, I don't want to hijack with big pictures)
You're not gonna want that much coverage though. I tried technique #2 but the paint dried too fast so went with technique #1. My saran wrap was angles, lines and curves. Your sponge will be circles and dots. With some patience, it should turn out pretty good.
I am interested in seeing how it turns out. I like the idea of having the snow accumulating around the bottom, maybe on the leading edges of a fin or two.
Good luck,
Mike