One of the easiest ways to add realism to your models is to paint some simple chips to break up surfaces. Just about everything in the real-world has imperfections on their surfaces, and some things like shipping containers, construction equipment, old vehicles, dumpsters, etc. are more worn than newer cars, for example, so vary the level of chipping to match real world examples of the same subjects.
A brush can be used if a paint chip needs to be placed in a specific spot, but if accuracy is less important and random placement of the paint chips is called for, the fastest and easiest way to add chips to your models is with a sponge. A great, very inexpensive way to do this is with a torn piece of packing sponge that often comes in figure blister packs or pluck foam from your figure cases. Using a clothes pin to hold the foam makes for an easy to use tool.
For this quick tutorial, we're working with some sci-fi crates that were designed by Imperial Terrain and printed, painted, and weathered in the Tabletop History studio. The photo below is some work in progress after paint chips were added using the easy method described below.
To have a fully functioning studio, a lot of tools, equipment, and consumables are needed, but the best tool for making paint chips is a cheap addition.
The method to make believable, easy paint chips is very similar to a drybrushing technique, except it's a dabbing motion not like a paint stroke motion. Dip the sponge into some wet paint. (For this method and regular drybrushing, I don't use my usual wet palette; instead I use a ceramic tile as a dry palette.) With wet paint on the sponge, dab the paint onto the palette to remove the bulk of the paint.
Next, dab the sponge onto a paper towel to remove most of the paint from the sponge.
Testing the sponge on your thumb is a good way to check that you have the right level of drybrush.
Starting with a primed, painted, and dry model (in this example a crate), consider where the real-world version of the object would get scratched and chipped. This is usually on the corners and edges and less often on flat surfaces.
Give this easy method a try and add some simple realism to your models. Good luck and good modeling!