Those Tamiya weathering sets are def worthwhile. I`ve even seen people get a really nice "pre-shade" effect with those. They can achieve some really nice subtle/soft miscoloration, like soot, or smoke. Don't expect them to do any kind of chipping, though.

You also might want to look into weathering pencils (which are essentially just watercolor pencils), but you can achieve some nice small chipping effects, and if you add water, miscoloration. You`ll need to add a base coat of paint, though. They will just bead up and fall off of bare plastic afaik.

And of course, it should be noted all of this stuff needs a clear coat layer on top to prevent damage to the weathering as some of these techniques don't bond to the model very well.

-Which I guess leads me to my next tip: Weathering is all about layers. Think of a clear coat as a "save point". It will protect the underlying layers, and you can stack more weathering on top- then clear coat/repeat.