How do I stain over paint?

You need to use paints , stains & primers that are compatible , water based and oil based won't work together , either leaving it tacky , or not sticking or peeling .

Stain and paint are two entirely different things and should not be mixed. A stain will not work over a primer. Stain is for staining raw wood. If you want to stain your mantle, remove what you have on there now, sand and stain.

To create a wood grain effect you need to buy a Small piece of wood to stain it as you like so you can color match within a range of realism. Select your Stains wisely, and you need to apply Stain over a usually darker beige or tan base paint with either:

A. a Rag rather than a Sponge or Roller.

B. a Textured Roller.

C. For large Grain splotches, your thumb in a Glove.

When done, excepting instances where the effect is applied reverse, (by staining glass to adhere glass to the woodwork or shadowbox or otherwise plyboard a.k.a how to get a Perfect Sheen and Perfect Clean every time), you will need Polyurethane or Varnish or flexseal clear to protect the Stain from chafing, and dust.

Stain over paint takes a long time to dry. Stain is made to absorb into bare wood so it doesn't really absorb into the paint but it will dry on the top of the paint. You would have to put a poly finish over the stain that covers the paint or it will come off as you clean. I have used stain over paint to antique DIY signs but never a large piece of furniture. I am not sure if you stained over the primer or you added another coat of paint over the primer. You could use an chalk paint wax--clear + a darker wax to give you mantle more of an antique old looking appearance. Or you can dry brush a darker color(s) over your paint to make it look more like aged wood. https://www.hometalk.com/diy/living-room/furniture/dry-brush-old-wood-technique-7554861?expand_all_questions=1

Dry Brush Old Wood Technique