dthurman said:
Can you tell me what the advantages or disadvantages are in using oil pastels?
David, I've only ever used them in a "fine art" capacity, working on heavy paper to make drawings. I don't have any experience using them for weathering. However, because of my experience with them I wouldn't use them for weathering. There are other mediums which, in my opinion, are superior to oil pastels.
If you want the flexibility of using oils (endless color possibilities, finely ground pigments, thin finish), then artists oils are superior to oil pastels. Artists oils can be applied to any surface with brushes or just about any other tool you can think of. Oil pastels would have to be ground into a paste to accomplish the same thing.
You can sharpen or shape oil pastels into a pen and mark directly on any surface with them, but their softness necessitates continuous sharpening and reshaping to keep them in a usable shape. Typically, you would rotate them in your grip as you draw with them (much in the same way you rotate a drafting pencil as you draw) to keep them shaped into a point, but given the precision required for weathering, especially in N scale, what works for a portrait or landscape drawing, might not work as well for drawing on rust streaks or spattered mud. However, if a pen is more your speed than a brush, you might have better success with colored pencils.
While the comparison between pastel chalks and oil pastel crayons might be the easiest to make, they are actually totally different mediums and do not behave in the same way. In other words, since most modelers who use pastel chalks for weathering grind them up into a powder, and this cannot be done easily (if at all) with oil pastel crayons, there really is no comparison.
A better comparison to pastel chalks would be weathering powders, such as those offered by AIM. Their weathering powders have an affixative blended into the powder, so when they go on, they stay on. While this aspect is appealing to many people, especially because pastel chalks have a tendency to "disappear" when you clear coat them, I am not comfortable without having the ability to undo any mistakes. I'd rather build up layers of chalk and "underweather" a model than put one too heavy application of weathering powder and "overweather" a model.
As I've more or less stated, my current methods involve oil paints and pastel chalks. Obviously, there are many methods and many folks who do exquisite work with other mediums, but these are the two that I like best. I use other techniques, too, and while I haven't tried everything, I've found a few things that work for me.
In this example, I mixed chalk with mineral spirits and dragged it downward from the roofwalk supports in an attempt to copy a prototype photo. I piled some ground chalk on parts of the roofwalk and wet it with mineral spirits to get a chunky glob effect to simulate the nasty, rotting chunks of spilled grain that collect under the roofwalks and around the hatches. I also airbrushed the grime "spray" that seems to collect on the underframe using enamels. When I get this model back out of storage, I'd like to spend some more time on the underframe to get it away from looking so plain. But, it's a good illustration of the chalk/mineral spirits technique, so here it is:
This model shows artists oils painted directly on the model. For rust spots, I paint the lightest shades first then build the rust spot toward the center using progressively darker shades. Many of the rust spots are actually real rust, embellished with paint. For the real rust, I took steel shavings and sprinkled them over the model after I spritzed it with a 50-50 mixutre of hydrogen peroxide and water. I stored the model in an airtight bin, opening it daily to mist it again. I did that for about two weeks until some pretty good rust spots appeared. I also applied titanium white directly from the tube onto the carbody to tone the whole car down a bit. I worked the paint with a large hogshair brush until the paint was spread evenly but nearly gone leaving a whitish cast, which seems to simulate the dusty appearance of some prototype cars. Also, it's worth mentioning that on this car, I painted the insides of the stirrups a dark brown/black color to make them appear thinner. It seemed to work well, so I've started doing it on all my cars.
This model was weathered using 1000 grit sandpaper (to remove the lettering) and chalks. It's a pretty straightforward weathering job, but for as little effort as I put into it, it looks the part.
I hope this gives you some ideas of what can be done with a very simple palette of materials and colors. If I can suggest anything, it's that you try everything until one method "speaks" to you. Master that method and then you can apply what you learn from it to any other method.