I use emery cloth sandpaper if needed, But usually You won't have to sand it at all because
it has already been sandblasted. I just cut it and start soldering.
If you want the polished look you can sand it down to a shine while it is in pieces with any
fine grit sandpaper or emery cloth.
I use silver solder and regular rosin core solder, obviously silver solder is best and stronger.
I always use "Lucky Bobs" flux. Use at least a 40 Watts iron.
Because it has already been sandblasted and the kind of steel it is, it will solder fast and easy.
I have never had any problems with it holding fast, and it is always strong.
Now after building it you can go over the solder joints where it might look burned or brown
with emery cloth or fine metal sandpaper, just to clean it up, or to shine it up. 
Denny.

Uncle Gary using the instructions above.

First I set the Chassis in my jig, I know you don't need to use one, but I felt it would help me in the build. The material seem thin, by using the jig it would make it easier for me to hold the sides in for on long solder.	I lined up all the pieces with the bushings and axle in place.
As you can see the ends of the sides wouldn't line up due to the thin material.	I begun in the rear by tacking the end of the side rails in place.
Then I tacked the motor mount and side rail for the next move. The material gets hot so be careful.	One long solder from the motor mount to the end of the side rail, don't overload it with solder but make sure the solder is hot enough to flow some under the two piece you're connecting.
The front extension was tacked.	Tacking the front side rail at the motor mount.
Motor mount soldered before the front sire rails.	Motor mount and side rails tacked,
I ran the whole front side rail like i did for the rear side.	Make sure you don't overload the solder, a little bit goes a long way if you get it hot and let it flow, it's good to have some overflow under the side rail.
Front extension and front wheels mounts were just tacked in.	The final product, I thought before the built that the chassis was too thin and it might have too much flex, the soldering of both side rails did strengthen the chassis, the only weakness would be the front body mount, this will be done by Mr. Motens' team.
This complete chassis is going to go to Mr. Motens class for them to mount a body, I think this chassis should be build by an experienced builder that understand heat and the flow of solder, if a novice attempts to build this chassis, the builder might run into a few issues.	Dennis it was a pleasure to build this chassis, I'm glad I was able to help on this project. DVan gave the chassis #4 to build for myself and I'll let you know how I mounted the body, Thank you. Uncle Gary