HI all here is my method of ageing plastic (as used on the 1/6 Tamiya bike airboxes.
This is what I use

as you can see I just daub it all over the part I want to age.
I then get my dirty white spirit bottle

and I flood the part with the thinners
then whilst its still wet I flood it with my wash

then just let it dry, and there you go instant aged plastic


you can add more washes or rub the ageing off if you think its too much then you juat fix it with either matt semi matt or gloss varnish ( or whatever your chosen finish is)