Strip to bare wood.
Clean.
Prime.
(Sand if desired.)
Paint.
I have never had satisfactory results when using any type of paint over the top of factory clear sealers (urethane, shellac, or other).
I tested six different clear coats for my last painted project. The one that I liked the best, and has held up the best, was Rust-Oleum
'Painters Touch' 2x Ultra-Cover matte clear. On that stock, the clear was used over a textured paint - Rust-Oleum 'Enamel Multi-Colored Textured' in "
Desert Bisque".
Personally... I'd probably just sand off the factory finish and hand-rub some coats of thinned Tung oil. If desired, the dents and dings could be ironed out before the Tung oil applications.
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This one went the opposite direction. I stripped the paint and refinished the original wood.
This rifle had 2-10 layers of paint on it, depending upon what area you were looking at (there was a previous camo paint job under the pink - and the barrel had many layers).
The stock was also quite badly abused. To make matters worse, the pressed checkering on the fore-end was full of enamel paint that I could not remove.
So, a wet towel, a good iron, and some patience were put to work. I ended up being able to raise every dent and ding in the stock.
I then spent about a day raising, sanding, raising, sanding, raising, and sanding the pressed-checkering.
The result was a visible checkering pattern on the stock, but you could not feel it, whatsoever. My brothers and I called it "Ghost Checkering".
Some hand-rubbed coats of Tung oil brought that old stock back to life, after the work was done.
It went from decent 'deer rifle' to battered 'truck gun' to ... well, I'm not sure ... and then back to something respectable. Last I heard, it was to be a 14 year-old kid's first deer rifle.
(Mossberg 800BSM 'scoped police' prototype, circa 1973, built from left over parts spanning 6+ years of production.)