MasonJar has given you good advice.
I would hold off on the "large flat file". Use some #400 grit wet-dry sandpaper (used wet) on a piece of plate glass, otherwise you will end up removing too much material. Resin sands very quickly, and files even quicker . . .
Get hold of a piece of flat steel, even a cookie sheet will do, and use rectangular magnets to hold parts in place, and keep them square, while you build the beast.
The catalyst used for polyurethane resin contains isocyanides. That's iso-CYANIDEs. When you're sanding it dry or filing it or milling it, wear a mask to avoid inhaling particles. Your wife and your grandchildren will thank you. Better yet, sand it wet, don't file it, and that will eliminate the dust problem. Clean up afterwards.
Sometimes you will encounter resin parts that aren't straight, or flat. Run them under a hot tap, between 180° and 200° F. The resin quickly softens with exposure to heat, and you can then bend parts to their proper shape.
Once it has cured, polyurethane resin is impervious to hydrocarbons, so you can use any type of paint on it, and you can't glue it with conventional model cement. You do need to clean it before painting. Most manufacturers use a silicone-based mould release that does a very good job of keeping resin from sticking to RTV, and keeps paint from sticking to resin. Warm water and Sunlight will remove it.
The main thing is to take your time. Make sure the parts are flat. Make sure your joints are square. Wash the parts before attempting to glue them.
Finally, know that preparing parts for resin casting is difficult. Casting parts in resin is also difficult. Casting parts in resin is also very expensive. If you're really good, you'll only suffer a 20% failure rate, so you'll only waste $10 for every $50 worth of material you buy. And while I don't know about what Walthers has set up, I do know that 99% of resin kits are produced by individuals working in their garage, or basement, who spend time away from their families and their own hobby interests, and who rarely do more than break even on their efforts. That's why they don't last more than a few years, if you've ever wondered.
How do I know this? I've built many model of aircraft and tanks from vacuform and resin kits. I've poured gallons of resin. I've designed complete kits and aftermarket accessory kits for same. Now I've moved in to the realm of trains, designing photoetch, resin parts, and explorig stereolithography as a means of creating masters. I keep getting sucked into it — I have a dozen projects in front of me, but it gets my creative juices flowing, and I end up spending my time on new products rather than building stuff for my own layout. I need to learn to not answer the phone.
Anyway resin kits are simple, except that you have to use CA rather than styrene ement. Don't be shy. Build a few, and a whole new world will open up to you.
Cheers
Scott Fraser
Calgary, Alberta
