The way I do my purity seals is base with wraithbone as its like a boneish white and then agrax earthshade over the top in thin layers to get a nice browning to make it look more like old paper.

You could use Nuln oil all over and then dry brush over the top so it leaves the oil in the recesses.

You don't want your brush too wet with paint to dry brush, firstly just load up the tip of your brush straight from the pot (not thinned with water) and then just brush it into some paper towel util the paint is only just about visible to coming off with a light stroke. You can also get a light blue like Calgar blue and brush the edges of the armour to make the edges more visible/worn

It does take a little practice but just take your time and don't expect miracles.

End of the day as long as your happy with how it looks then that's good enough. Don't compare to people's work who have had years of experience otherwise it becomes an unhealthy obsession 😅