Ed's got it with sifting real dirt. Sometimes it is best to bake it for 10 minutes at about 200F. This will kill any germs/bugs/thingies that might be alive in it.
Put a brillo pad in a jar, cover the pad with water and let it evaporate (about 30 days). Grind it up...Real rust!
Usually when I decant a can, I will hold it upside down and spray until the pressure is fully gone. Then I puncture a hole and drain the can into a jar. I have never had a problem doing it this way.
Please don't be discouraged by the comments of customcreator. I believe he was trying to help you with his criticisms but may have come across a bit obtuse. We have all been guilty of this at one time or another. No one here is trying to discourage you from building dioramas, me included as we rely on young builders like you to carry the "diorama torch" when all of us older guys are gone. One thing that many of us don't do after critsizing is giving alternative ways of doing something. I'll try to do some of that below.
Try to "Let it roll" as they say 'cause if your temper gets out of line, as a 13 year old, your blood pressure will go crazy and you won't be building anything for much longer. LOL!
You have some good things going on in this diorama. It looks as if your ability of using your minds eye to get the right size for building in 1/25 scale. The door sizes, walls and windows, for instance, look to be on the mark. The fact that you may have taken the time to trim the curved ends from the popsicle sticks and butt the ends together for supports says you have the right idea. Using less glue and painting the supports before gluing them to the insides of the wall will be more realistic. It looks like you are using a glue gun with those glue sticks. Those are very hard to work with as the glue glops and dries too fast. White glue or wood glue is better for paper products. It is not expensive either as for a 13 year old I am sure you are on a limited budget (I know I sure was). Your models, tools, furniture and machinery in this dio look very cleanly built (no glue globs) and may benefit with a powder of chalks or pastels to tone them down a bit to make them look more "used". If they are diecast, a blackwash will help the detailing stand out.
You've obviously got skillz for making the right size signs and for making workable hinges on your swinging door on the fence. Using India Ink diluted with water will help in "aging" your fence and making it look weathered as anything exposed to the weather would do.
Dukefan, there is a ton of experience on this board and other diorama boards on the net. If you have a question on how to build or make something from scratch, please ask as there are several people on this board who will suggest ways to make them and give ideas.
I also suggest you visit the diorama forum at this link as the best diorama guys in the country hang out here:
Beautiful job on rusting that truck. Very realistic and nicely done in 1/18th. Ummm, as for the other two, I believe they were fiberglass bodies and would not have been found rusty but probably broken or cracked with little fibers sticking out of the cracks. The dust is very realistic for the cars sitting in the corner of a yard for a long time. Nice work!
As the guys have mentioned, G scale railroad or 1/2 inch dollhouse supplies are about the only way to go for storefronts even close to 1/25 scale. They can be rather expensive though. Your best bet is to scratch build what you want.
If you decide to go that route and need some help, please ask here as there is a bunch of people that are willing to give you their input.
Let's see some progress pictures as you go...good luck!
There are several ways you could simulate a concrete floor. It would depend on how much texture you would want . Pouring plaster or hydrocal into a mold made with balsa or styrene is a good way to start. One thing I like to do is to spray a piece of posterboard or thick styrene with gray primer, sprinkle baking soda onto the wet paint, shake off the excess, spray again with gray primer, add some gray, white or/and black pastels for shading and voila! a realistic looking concrete floor.
Here is a pic of a floor I did using this method. The base was plastic. I scribed the slab lines using the back of an exacto.
Here some oil stains were added for realism.
You can play with the amount of texture you want on your floor by adding more or less baking soda:
This has a heavier coat (or two).
making the floor look heavily used by adding dirt contributes to the "texture".
Hopefully this will spark some ideas for making your own floors J. Please ask any questions to help you through scratchbuilding the floor if that is what you decide to do. There is a wealth of knowledge in the diorama section here.