Painting - Painting garage door

My garage door needs painted. It is about 52 years old and it shows because of the appearance. There are grooves where the paint as come off of the years and then has been repainted. Is there a product that can be used to fill these areas (there are a ton of them) and then can be sanded to create the appearance of the wood being flat before being repainted? (Like how you can fill holes and sand drywall with joint compound)

Thanks!

You could use 'rock hard putty' [made by durham] You can apply it similar to drywall mud but it is a bear to sand. I would rather use too little than to much.

I would get a disc sander. Prime the whole door then with a putty knife fill the low spots with exterior spackle (I favor Synkoloids), let dry. Power sand the surface with 100g with the disc sander. If that takes it off too fast to keep it smooth for you try an orbital sander. Then reprime spackled areas and put on finish coat.