Rather than layering up the paint and obscuring finer detail...
You could strip the paint off and start from clean.
I see you're in the
UK, Over here Dettol Antiseptic Liquid seems to be the easiest thing to get hold of.
Asda and Sainsburries both carry it, its the original browny orange liquid that stinks like hospitals that you want.
Leave a figure (plastic, resin, metal its all fine) submerged in pure dettol liquid for a good 24 hours or so and the paint just scrubs off with next to no effort with an old toothbrush
A couple of baths in clean dettol each time gets them fresh as a daisy, then rinse them off in water (important; only use water once they are free of paint cus it sticks back to it)
I've recently stripped a load of mairines, you can see the results in a thread called 'reconditioning my marines...'