First things first... while the term "resin" gets used colloquially, realize that ALL polymers are technically resins, including the polystyrene which injected models are typically made from.
What you're typically talking about with "resin" kits is polyurethane.
Plastic resins are divided into two classes... thermosets and thermoplastics. The difference is how are the materials "polymerized." All plastics are made up of little molecules called "mers" which form chains with other mers to form polymers.
Thermoplastics form chains easily, but the bonds forming the chains also break easily with the application of temperature. In other words... thermoplastics MELT.
Thermosets, on the other hand, have much stronger bonds which typically require some special techniques to form. Some thermosets require the application of high external heat in order to polymerize. Some can work at room temperature with the addition of a polymerizing agent.
The plastics used in electrical hardware (wall outlet cover plates, for instance) are thermosets. They won't MELT, but they'll burn. You may have seen this. Epoxy is also a thermoset, but it's the sort that forms its bonds when mixed with a polymerizing agent.
What is usually called "resin," polyurethane, is a thermoset as well. You need to mix the two parts and then pour them into a mold. The advantage is that you don't require high heat or high pressure, or even a particularly robust mold. The disadvantage is that it's harder to work with and if you screw up, the material is wasted forever.
So for "garage kits" a guy makes a "pattern" piece (often of wood, or of epoxy putty, or whatevr) through basic sculpting processes. He then makes an RTV (room-temperature-vulcanizing rubber) mold off of this pattern. The rubber mold can be poured full of the urethane resin and it makes a nice copy of the original part. It's time-consuming (hours per part versus seconds for injection molding) but it's something that ANY of us can do without spending a fortune.
The trick with thermosets is that they're very resistant to the breaking of their bonds, either through temperature (they burn, they don't melt) or through the application of chemicals (again, they can be corroded, but not "softened" like polystyrene, for example, can). So you can't use the same tools for them that you would for an injection-molded object.
Since the original mold is almost always RTV rubber, the mold is liberally coated with a mold-release agent (a very slick oil) prior to the casting of the part, and that's always still present on the part when you get it, sometime heavily, something just a little bit... but it's enough to ruin a build-up if you don't take precautions to take it all off. I recommend a long soak (several days) in a strong solution of pine-sol. Other people use oven cleaner or other compounds... it's really a matter of choice. I soak for days, then scrub with a toothbrush, then do a final wash using Dawn or something like that, then air-dry.
Solvent-based cements won't work, for obvious reasons. Cyanoacrylates work nicely, as stated above, but use the long-cure stuff... if it bonds "wrong" you've just lost your model! I recommend buying both the long-cure cyanoacrylate and some "superglue accelerator" (realize that superglue is also a thermoset polymer!) which is usually in a spray bottle. Put a layer of the superglue across the bonding surface, press the two parts together, hold, wipe away any excess, then (when you're SURE the parts are aligned right!) spray with the accelerator. (Note that it will actually get pretty hot once sprayed, and will cure almost instantly.)
One thing to remember... the superglue is actually tougher than the resin, so try to avoid having the glue outside of the intended location.
I've found that gap-filling cyanoacrylate (superglue) works very well for filling in gaps instead of using putty. I do most of my filling with superglue, and use the putty only for the very final "polish" layer. Once it's sanded perfectly smooth, I sometimes will apply some water-thin superglue over the surface to seal the pores in the putty. Note that I do this wearing gloves, never touch the stuff with my hands, and work on a DISPOSABLE SURFACE, covered with waxed paper. And with LOTS of ventilation (the vapors from cyanoacrylate are actually fairly hazardous!)
The advantage of this is that you get, effectively, a model made of a single continuous piece of plastic with no real "joints" for all practical purposes. If you drop the thing, assuming that you had CLEAN PARTS, if it breaks it'll probably break somewhere away from the joint, since the joint will be stronger than the rest of the model!
Now, for painting... the key to painting resin kits is PRIME-PRIME-PRIME. Don't assume that your first layer is the final one. I suggest automotive grey primer. Apply thin coats, look for pores or flaws you couldn't see before, putty and sand... apply another coat... lather, rinse, repeat. It usually takes me three or four primes before I'm happy with the finish. Key to this is to sand using automotive-body-quality super-fine-grit sandpapers, not the "woodworker" stuff you may be used to, and to WET SAND (meaning, literally sand with the surface wet, which lubricates the sanding operation and also washes away the sanding debris). It'll take a long time but it's worth it. The goal is to have a very very smooth, flawless finish.
Then, and only then, do you do the main paint job, on top of your primer layers. Since the model should be a nice uniform gray tone, the coat on top doesn't need to be tremendously thick. I recommend using an airbrush and using laquers, which tend to form a harder surface than enamels, but that's personal preference. You can use a brush to do fine details, but for any areas large enough to show brush marks, I never use a manual brush... I airbrush everything.
As with anything else, it's better to go with multiple thin layers of paint rather than "slathering it on." You'll get a much better-looking model. It also leaves the edges from your masking (use low-tack masking tape!) as thin as possible... which is strongly desireable from an appearance standpoint as well.
Patience is the key to a good model. Rush it and be prepared to pay the price!
EDIT: About using flats versus glosses... it depends on your style. Flats tend to be easier to work with, but I like the harder, smoother finish of glosses (and with laquers, the drying time is fast enough that you don't have to worry about the pitfalls of having a glossy surface that's "wet" for a long time and ends up collecting dust or bugs or whatever!)
If using enamels, use flats, then "seal" with a several coats of gloss clear laquer before applying decals. You NEVER want to apply decals over a flat surface! If using laquers, go gloss all the way. You can "flatten" it down later with a final coating of semigloss laquer over the decals... you want to overspray decals anyway!
