As to the flat/glossy question-- I don't have any experience there, but I don't really think it should matter much... that said, I'd stick to flat. Flat SHOULD have a higher solids (and probably slightly larger particles) count and therefore cover up and fill in stuff better. Just stay away from the stupid "paint and primer in one" materials... Those are just counter to everything that needs to be done to get a good finish-- they're a "one step" product for lazy guys, IMHO... the whole purpose of priming is filling in minor imperfections and the surface being sanded down smooth PRIOR to paint... How do you sand the primer if it's IN the paint?? Plus, you can't put a heavy load of primer solids in the paint without compromising the color and consistency and applicability of the paint, yet the flowability of the paint is hampered by the solids of the primer anyway... Yechh-- what a mess... stuff can't do EITHER job right IMHO...
No, the finish of the primer has no effect on the final "finish" of the paint (meaning whether it's glossy or flat finish paint). The primer's job is to transfer as much "solids" (particles in a solvent) to the surface being finished as possible, to flow out and fill as many imperfections as possible, to promote adhesion between itself and the surface being finished, and promote adhesion of the paint layer going over it to itself and the surface, and to be sandable enough to allow the surface blemishes to be sanded smooth to hide their presence. Paint, OTOH, is simply pigments in a solvent designed to provide color. PAINT DOES NOT HIDE FLAWS, IT MAGNIFIES THEM!!! Whatever your surface looks like when you're done priming and sanding, is about what it'll look like when it's painted; if you can see imperfections in your primer coat after sanding, you WILL see them after painting. Many people mistakenly believe that paint will hide imperfections, that even if they can see imperfections after priming and sanding that they'll somehow 'disappear' when the paint is applied, that "the paint will cover it up". This isn't true. BASICALLY, THE APPEARANCE OF THE FINAL FINISH IS HOW IT LOOKS BEFORE THE PAINT IS APPLIED... IOW, if you can see imperfections, flaws, etc in the sanded primer BEFORE you paint, you will almost certainly see them AFTER painting.
"Wet sandable" primer is designed to be sanded using water. "Dry sandable" primer is not designed for this. "Wet sanding" is a process used to make professional-grade finishes on things like show cars and stuff like that, where you want that "wet look" or "mile deep" paint job... Typically it's done by using a garden hose just dribbling water on the surface being sanded while the surface is sanded with a fine-grade sandpaper using a long sanding bar to get the surface COMPLETELY SMOOTH and level and even. The purpose of the water is to cool and lubricate the sandpaper and to flush away sanded off particles to prevent these very-fine-grit sandpapers from clogging up with particles. Dry sanding liberates a lot of particles in the form of sanding dust... wet sanding takes those particles away in water. Sanding also creates heat via friction, and can make the primer sticky and gummy, gumming up your sandpaper and clogging it with particles, making it worthless. Wet sanding can carry this heat away, prevent the gumminess, and flush the particles from the paper preventing clogging of the paper.
Now, obviously using true "wet sanding" principles with a dribbling garden hose water source isn't practical for most model rocketry applications (particularly paper tubes and wood fins, although fiberglass and other waterproof materials certainly CAN be wet-sanded) due to the nature of the materials (paper tubing and wood fins/cones). BUT, we can use a variation of the method that produces near FLAWLESS results... I call it "damp sanding". I start by hardening my balsa cones and transitions with ultra-thin CA glue wicked into the surface... take it outside (so the fumes don't run you out of the house) and wet the surface of the cone/transition with ultra-thin CA glue, which will wick itself down into the wood grain of the balsa like a sponge soaking up water. There it will cure and "harden" the balsa, much like fiberglass impregnated with resin. This helps toughen the cone so it's not AS prone to damage (as an unhardened one). Doesn't make it "bulletproof" but it DOES help... Sand down the "grit" and "fuzz" raised by the CA with 220 grit paper, then I brush on a layer of Elmer's Carpenter's Wood Filler thinned down to about the consistency of hot dog mustard with a few drops of water worked into a dab of it in a small disposable bowl with the brush. Let this dry a couple hours, then sand it down with 220 grit paper followed by 400 grit paper to fill and eliminate the wood grain and large surface imperfections. Then I give it 2-3 progressively heavier coats of primer, allow to dry fully (overnight is best, longer depending on your climatic conditions at the time, but I HAVE sanded within just a few hours on occasion...) Sand with 220 grit paper followed by 400 grit paper, dry. Carefully inspect the surface, looking for any major imperfections such as low spots, waviness, high spots where the primer was sanded through to the bare surface, etc. If necessary, reapply more primer and allow to dry and sand again... though usually this isn't necessary if you'd done things right up to this point. Sometimes just hit a light coat of primer on any 'high spots' where the primer sanded through. Now, you're ready to either 1) call it good and paint or 2) damp sand.
To be continued...
OL J R
