Sure... glad to help if I can.
Here's how I do it. Once I have the rocket constructed and I'm ready to start finishing it (any filling/rough sanding all done, or fins papered, etc.) then I shoot the rocket with a couple good coats of a sandable primer. I'm kinda fond of the regular old Walmart Colorplace primer myself, because it's cheap and it works well. Of course IF you're going to use a lacquer based paint, or a different brand, it's always best to use a lacquer primer or the same brand and type of primer as the paint brand and type you plan on using, for compatibilities' sake.
Once your primer is dry, start by sanding the rocket DRY with 220 grit. I 'free sand" (without a sanding block) as it's easier for me-- I've done a lot of car sanding and farm equipment sanding and I'm used to doing it that way, but most folks get better results using a sanding block). Thing to remember with sanding, is ALWAYS KEEP MOVING, NEVER SAND IN JUST ONE SPOT, and sand in CIRCLES and NOT STRAIGHT LINES if you can help it. Sanding in a straight line tends to cut grooves into whatever your sanding and leave deep sanding scratches. Another important thing to remember is, LET THE PAPER DO THE WORK. Don't use a lot of pressure or "push down" hard on the paper-- light pressure works just as well if not better, heavy pressure only clogs your paper, makes sanding scratches, dulls the grit of the paper, and generates heat which makes the primer gummy. Make a couple passes around the rocket, sanding the whole thing. Hold it up to the light and let the light "glint" off the rocket, and wipe it down periodically with a paper towel to remove the sanding dust. Look for any obvious high spots, low spots, or imperfections, and sand them as smooth as you can. It's likely you'll wear through the primer and down to the original glassine coating on the tube in places... but you should have SOME primer left on the rocket when you're done... If not, you probably sanded too much. Reprime any bare spots if needed and resand them to get them smooth and even. A few bare spots are ok for painting over, but it's better to have some primer under the paint, so repriming the bare spots is a good idea.
NOW, we come to the wet sanding. I actually prefer to call it 'damp sanding', because, unlike the wet sanding typically done on cars, you actually need FAR FAR less water to do a good job, and as you said, with the absorbent and rather delicate nature of things like cardboard tubes and balsa fins and nosecones, which tend to warp or dissolve in water, you DEFINITELY don't want to use too much water. You only need enough water to keep the sandpaper from clogging up.
I get a small bowl of water (like an old Tupperware or something like that, preferably retired from the kitchen and used 'for sanding only' since you WILL get primer or paint particles in the bowl from the sanding process. You'll also need some paper towels, a regular old cloth towel (worn out bathtowels are handy) and some 600 grit wet/dry sandpaper. I usually start sanding dry, just to take out any sanding scratches or other stuff that needs a little more dressing from the 220 grit sanding before I start. When I start damp sanding, I dunk the sandpaper in the bowl (I usually sand with a small piece of paper about 1.5 by 3 inches or so) and then 'shake off' the excess water. The first time you'll probably have to rub the paper with your fingers a bit to get the water down into the grit of the paper. Shake off the excess over the bowl, and starting with the dry rocket, start sanding in small circles, gently turning the rocket constantly in your hand as you do. I start at the top of the tube and work to the bottom, sanding about 2-3 inches of the length of the tube at a time, all the way around, and then moving down to the next section of tube below it. The sandpaper will start sanding off the primer dust and mixing it with the water, leaving behind a fine 'sanding mud' behind the paper on the rocket. The more you sand, the drier the paper, rocket, and sanding mud will become. Periodically dunk the paper in the water and shake it off and sometime daub it on the old towel to remove excess water. Look at the grit side of the paper, and if it starts to get clogged (paint 'spots' clinging to the paper) rub it gently underwater to release the trapped paint. Periodically wipe the 'sanding mud' off the rocket to get it out of the way. You really don't want it to dry on the rocket, as it's hard to get off and messy and defeats the purpose. Things don't have to be sloppy wet (water dripping off the rocket) but it does need to stay moist-- usually a drop of water from a finger dipped in the bowl will give enough water to continue sanding if the sanding mud starts to become more of a 'paste'.... but then it's probably about time to rinse and clean the paper again anyway, and wipe the sanding mud off with a paper towel.
It's surprisingly easy to do... I wash the paper about every minute or so of sanding, and wipe the rocket down about the same. Damp sand the whole rocket, then hold it up to the light and let the light 'glint' off it, and look at the reflected 'stripe" of light. If it's smooth and even, you know you're there. If you can see imperfections in the reflected glint of light, like pock marks, scratches, dimples, holes, broken or unstraight 'lines' of the "glint" of reflected light, then you have low spots, high spots, unfilled pores or grain or dimples or uneven-ness and depending on how good a finish you want, you may want to go back and correct these problems now. Going back to using some spot putty to fill a low spot means starting over (at least on that spot, as you'll have to resand with 220, reprime, resand again with 220, then re-damp sand with 600 grit, but if you want a flawless finish, that's what you need to do, and NOW'S the time to do it! Once your satisfied with your sanding job and the surface appearance, use a pretty damp paper towel (wet but not dripping-- dipped and squeezed out pretty good is about right) to wipe all the remaining sanding mud off the rocket, fins, nosecone, etc. (actually I usually do the nosecone seperate-- it's just easier that way)
Wipe the rocket down with a clean dry towel to remove any residual moisture, and let it air dry thoroughly. If you want, you can clean it again with a tack rag to remove any residual paint dust before you do your color coats...
If you do it right, you CAN make primer actually SHINE. Getting a great finish on a rocket is 95% in the prep work-- the better you do at filling, sanding, priming, sanding, damp sanding, and cleaning, the better the rocket will look when it's done. Using these methods, I haven't had to color sand yet and still I get very nice finishes. If you go to an automotive type finish, you may well have to do a little wet color sanding to really get the deep deep lustre that those finishes are meant to achieve... but that's usually the exception rather than the rule... and besides, the more work you put in ahead of the color coat, even if you DO have to wet sand the color coat, you'll have a LOT less color sanding to do to get a 'perfect' finish...
As far as the water is concerned, I haven't had any problems with damage to the tubes or fins. You don't want to use a LOT of water. The glassine coating on the tubes seals them from the water, so as long as you don't sand through the glassine (you'll know it if you do because you'll get an instant case of the "fuzzies"!) the tube won't absorb any water on the outside of it. True, primer IS slightly porous and will absorb a bit of water, but it evaporates readily because the primer coat is very thin. Balsa fins CAN absorb water, but if you used filler on them and have them coated with a good layer of primer, and don't use TONS of water, it shouldn't be a problem. I typically either paper my fins (most all) or wick CA into the fins and then fill and sand them; both methods seem to work fine with this method. When wet or damp sanding, you're not removing a LOT of material anyway-- the finer the paper the less material you're actually taking off. The idea is to SMOOTH the existing surface, not to remove large amounts of unwanted or unneeded material.
Good luck and hope this helps! OL JR
