How to fix a poor spackle job

I might consider wallpaper as an option. You can surely find it in a plain paper, in your choice of colors, or any of a million patterns. First, you would need to fill in the grooves with spackle, but now you can do it very quickly. No worries there, just ensure that there are no voids behind the paper. And a plain paper would be relatively inexpensive.

Another choice is to use a product that is closer to a latex caulk to fill the grooves. It stays semi-flexible (so less fear of cracking) and is paintable, and you can buy it in a tub form to spread with a knife. I used a product from DAP (sold as a crack filler) once to solve a similar problem, filling cracks in a ceiling.

A third option is to buy battens - thin strips of wood moulding, perhaps 1/4 inch thick, and perhaps an inch wide. You can even choose the profile for the moulding if you wish to give it a custom look. Nail these vertically over the grooves, and paint it all. You might also paint the battens a different color to highlight them. This will give the wall a completely different look, rather than just looking like painted paneling, and no spackle will be required.

1. Remove high spots by hand sanding the entire wall with a flat, rigid 1/2-sheet sanding block and 80 grit paper. It's ok to remove too much, but stop wherever you start to see the sheet rock facing paper or joint tape. Ideally, 100% of the wall will have been sanded.

2. For the areas (if any) where the paper shows before all the ridges (as in your photo) are gone, switch to either a 1/3-sheet or 1/4-sheet flat and rigid sanding block with 100 grit and sand broadly over the areas surrounding the show-throughs.

3. Remove the dust by wiping down the wall with a damp cloth/sponge kept clean by frequent rinsing in a large bucket.

4. Apply a thin coat of spackle using a 10" or larger metal blade. Apply less than required.

5. When dry, shave off any specs etc using the 10" blade over the entire wall.

6. Use a damp 4"x6"x2" sponge to 'sand' the new spackle, working the sponge from the middle of a patch of spackle outwards towards the edge and beyond.

7. Apply a second thin coat of spackle and repeat the shave/sponge steps.

8. Inspect for flaws using a bright flashlight or trouble-light held against the wall with the room lights turned off.

9. Touch up as necessary.