I was going to do some fender damage but it a cast metal tank so it won't bend. All the paints I use are water base acrylic. I have a big selection now compared to what I had when I did this one. I have a bunch from walmart and the rest from Michaels arts and crafts. The water based paints makes it easy to thin for many effects. The gloss colors make good oil and rust streaks and the flat colors make good mud and dirt right out of the bottle. Then to make it look dirty use the color you want for the wash and thin it out. I used a small water bottle about half full. Then put some paint on a brush and washed it in the bottle. Did this a few times and tested it on a old truck hood and let it dry. Came up with a mix that was easy to put on with the big brushes you can see in the first picture. Put on a coat and let it dry. Areas where more dirt would settle get another coat. Areas where the rain may streak it down the side get brushed as it drys. Keep doing it in small steps till you get the look you want. Take it slow and ad little details like boot prints. All the mud and spatter i did with small pieces of foam. Toys from yard sales make great things to practice on.
Jimmy