There are several ways you can make something look old and worn, and often you can get the best effect by using multiple techniques. Here are a few ideas:
– Using a matte coat over the mask will make it less shiny, and thus look less new. You can use a matte clear coat over any other weathering you do to protect it, as well.
– Painting on some weathering can get you some interesting effects, and is probably the most common way to make things look worn. Think about where a real mask like that would collect dirt, where it would be handled more and thus be worn down in those places, etc. Dry brushing on paint is a great way to make it look like the item has collected dirt. You can also use a paint wash around areas that would need greater shadow detailing to make them look less flat, and even use some stippling techniques to get a texture similar to that of the artwork. Try experimenting with a few techniques and see what works best for you.
– A crackle glaze can give a more ceramic-like look. This is especially effective if you mix it with a very small amount of a brown or grey, or if you use a very light wash of similar tones over it once dry and wipe it away, since this makes the crackle effect more prominent. The thicker you apply the crackle coat, the larger the cracks and the spaces between them will be – a thin coat can create small cracks that aren’t noticeable from afar except for adding texture.
– You can also try lightly dyeing the plastic with a polyester dye. Use a very small amount and short time as to not change the mask from noticeably white, but instead use just enough to make it look old and dingy. Grey tones and slight yellow tones work well for old plastic looks.
– Finally, you can abrade the mask with a bit of fine-grit sandpaper if you want visible scuff marks. Be careful not to sand too hard. This will work best if doing it before some of the methods that color the mask, since the color will collect in the abrasions and make them more pronounced.
I hope that helps! Good luck :]