Finishing castings requires color and texture. Look around and notice not only the color but also the texture of rusty metal or old wood. Floquil and Polly paints provide a huge range of colors, enough to accurately reproduce anything found naturally. The texture will come from powdered artist chalk. I use mostly Grumbacher, Rembrandt, Schmincke, and Sennelier brands. They can be found at most fine arts supply stores. Look for a starter set with a range of colors, mostly natural browns, greys, black, white, and orange rusts. Also try Dick Blick, an internet supplier of fine art supplies. (Search for "soft pastels".) Most sticks can also be purchased separately. The point here is to use high quality sticks with intense pigments. The problem I have encountered with “hobby” brands and powdered pigments is they tend to dissolve too quickly taking away the ability to create texture. This is the foundation of my weathering process!

The following list of chalk colors is compiled by color range. I have used these Grumbacher colors for years with consistently good results. Do not get bogged down with trying to find these exact colors! If you have several shades of brown and orange, with a nice range from light to dark, the weathering techniques presented here will work.

Please note these names are from Grumbacher brand. Other brands will have similar colors, just different names.

The Technique

When I set up to detail a group of castings I will open several jars of paint and have all of my chalk colors available to me at the same time, like a painters palette. This allows me to freely mix and blend colors. It is this ability to mix the colors at random that will create depth, shadows, and ultimately texture. Prior to beginning, drag a single edge blade along the chalk sticks to create powder. Scrape just a little at a time as needed from all of the colors. Ordinary rubbing alcohol is used to apply the chalk powder to the castings. It acts as a fair solvent in dissolving the powder partially and when it dries it fixes the chalk in place.

My castings are unique in two ways. First, I do not use a silicone mold release so there is no need to clean the castings prior to finishing. The surface has a natural tooth and readily accepts paints. Secondly, the castings are tinted and do not require a primer coat of paint.

Junk Piles and Workbenches

Lay all of the castings out to be painted. Many can be done at once. This method is much faster than one casting at a time. Add an appropriate base color to wooden portions. Use random washes of Floquil wood stains like Natural Pine, Maple, Walnut and Driftwood. Mix the stains freely and do not worry if you get the stain on areas of the casting that are non-wooden. Allow this base coloring to dry for a few hours. (Now that Floquil has discontinued their line of wood stains you may substitute 50/50 washes of Floquil paint and Dio-Sol. For Driftwood use Grime, Natural Pine use Rail Brown. Also try washes of Roof Brown, Earth, and Mud.)
Now open Floquil colors like Brass, Gunmetal, Old Silver, Graphite, Rust, and Dark Rust (a 50/50 mix of Rust and Roof Brown). Use a very small detail brush to paint a few of the tiny metal details on each casting with Brass, then repeat and paint several different details with Gunmetal. Continue in this random manner until all the metal details are done. Be neat and immediately scrape any unwanted paint off with the tip of a #11 blade. Next open Floquil colors like Reefer Grey, Depot Buff, Boxcar Red, Coach Green, Roof Brown, and paint the remaining details in the same manner. Add spots of color with red or green to any small bottle, can, or a random bit of trash.
Finally give the whole casting a general light wash of alcohol and chalk powder. Use light brown or grey chalk colors, just enough to muddy and dull the surface but not enough to cover over the painted details. Dip the tip of a soft brush first into the alcohol then into the powder. Gently pat the brush on a paper towel until most of the chalk mixture is removed then blot (do not brush) the small amount remaining all over the casting. Repeat a few times as needed, randomly selecting different chalk colors. Keep it subtle, all you want is a light film on the casting. In contrast, the following chalking techniques actually create the color and texture on the casting.

Described below are the two basic techniques I use to create rust and gunk. In addition they work quite well using different colors to produce say oily results (Polly-S Oily Black), or heavily weathered wood (brown/ochre chalks).
All of the castings are tinted. Try using the chalk techniques directly on some of them without applying any other color first. Allow a bit of the natural tint to peek through the weathering at random. This creates depth and ultimately shadows and texture.
For variety, apply a base coat using various colors of Floquil like Reefer Grey, Mud, Antique White, or any dull green, red, or orange. Then try the two techniques. (Stay away from bright base colors. Even though they can be found in real life they do stand out and look plastic on models.)

Rusty Texture

The first chalking technique applies a thin light layer of color and texture. Use this on slightly rusted object or where you want a lot of the original surface color to show through. By layering the colors you are creating depth. Adding the chalk creates texture. This combination of depth and texture on the flat non-reflective surface of resin is an unbeatable combination. The second technique applies a much heavier layer with more shades of rust and lots of tooth or texture.

Technique #1. Dip the tip of a small rounded brush into plain rubbing alcohol then immediately into a light shade of orange powdered chalk. (Remember, powder your chalk by scraping a single edge blade along the stick.) Now randomly blot this resulting mixture onto the casting creating a mottled uneven layer. Do not attempt to cover the entire surface. Use it full strength on some places and remove some onto a scrap piece of paper first for others. Now repeat with a couple of darker powders, on the same casting, using less powder as the colors darken. A very small amount of Burnt Umber (it’s almost black) can even be applied. Try to avoid using black as it is just too harsh. Keep the application totally random and mottled looking.

Technique #2. This method applies a much heavier layer of rust and works well on larger casting like junk piles. It will allow only a hint of the base paint or resin tint color to show through the chalk layer.
Use the tip of a single edge blade to scoop up and dump a small amount of dry powdered chalk directly onto the casting. Use different colors, dark and light at random, and mix them up. Now dip the tip of a 1/8" stiff square brush or a small round stenciling brush into plain rubbing alcohol, or for an even darker final color, dip it into the dark rust paint (50/50 Rust and Roof Brown). Then blot this directly over the chalk powder that was dumped onto the casting. Re–dip the brush often and make sure to move it around randomly. If you use too much alcohol or paint the chalk will totally dissolve ruining the effect. Build the color and texture up slowly. The most important thing to remember is to keep the chalk colors random and mottled.

Wooden Barrels and Crates

Use a dry brushing technique where you remove most of the color prior to brushing on the casting so the natural details and crevices on the casting work for you. This also allows a wood stain base or the tint to show through. Use a dark color like GN Empire Green or Roof Brown and very lightly dry brush the casting with a stiff flat 1/4" brush. This allows the individual slats of wood to show up but it does not add any color between them. Paint any raised bands or braces with the dark rust mixture. For extra depth use Floquil Grime and apply a very light dry brushing on top of it all to bring out the raised details, but just a touch. Finish them off with a  dusting of brown and grey chalk.