Drywall – Primer cracks once it begins to dry over new Drydex spackling

In my experience in painting my own house over 50 years I only sand when necessary, that is when the paint is loose, or, the surface is glossy. I just repainted some trim on my house that has Semi-Gloss paint. And, I sanded it first. I did this to improve the adhesion of the new paint. But, most of my house has flat paint, and I never sand it unless I'm blending an area where I had to remove some loose paint.

Update, 11/14/13: I re-read this today, including all the posts. It's interesting to see the different opinions about this subject. No wonder you had a question after searching the net.

My take on this is that if you are a perfectionist, you'll decide to remove it all. If you're just painting to resell, you'll scrape the loose off and paint. If you're going to keep it and use common sense, you'll scrape, sand where you need to feather edges for aesthetics, and paint. That's assuming its flat paint. For glossy finishes, you really better sand it to ensure the new paint sticks permanently. Incidentally, I always clean before scraping with Tide and bleach solution and brush hooked to expandable pole, and sometimes with a power washer to quickly knock off loose paint. Also, after sanding, I blow off the sanded surfaces with a blower, or brush it with a big paintbrush, or wipe it with rubbing alcohol or water.

Recently I used the tool below to "sand" the loose paint off my deck before repainting and was very impressed with it's efficiency. I like how it gets down into the cracks between the paint and wood and smooths out the ridges, as well as how quickly its use is. You might want to pick one of these up and give it a try.

Finally, one last tip. You can always do a test by painting a small area, waiting several days, then trying to scrape it off. That's how I learned how important sanding is for glossy paint.

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.