I recently purchased a cast scrap load for an HO scale 65-foot Athearn gondola. I was going to do a pipe load but when I saw the scrap casting I knew it had possibilities.
As purchased the casting was painted a solid rust color. Every item in the casting had the exact same color, which is not very realistic given the dissimilarities of the individual scrap pieces.
So to bring this casting to life all I had to do was to paint the individual scrap pieces with about nine different colors of acrylic and enamel paints, add some crusted rust washes to a few pieces and give the overall casting a dusting with several weathering powders. Very simple stuff.
Take a look at the difference between the before and after photos.