Why do sanded areas on walls show up after painting?

Due to the porous surface, going without primer will ensure that the spackle spots appear as a dull spot on the surface of the wall

An easy way to get around your problem would be to apply a 2x base coat to the walls and a final pre coat once dry ( leave 24 hours to set) start to apply your main coat x 3 applying a 4th coat if necessary.

Either you dry roll the wall, and again if you only touch-up on the area that's you sand , it will be difference,because you did not use the same paint that those developers used when they paint the wall. Plus there is some thick paint that you can use that will cover it properly.

Hi.
I think you use wrong grit of sanding paper and is scratch so now i can recommend to do the filling in the area where scratches are visible.
Ernest Rolnik Painting Services

Didn't sand it all down.

Well, after sanding you would have to put filler and let it dry and then give it a primer and finish with the paint layer that you like.

Is not sanded properly and anfter sanded you have to use the undercoat and then put the main paint.

Maybe try using a finer grade sandpaper

You could use a white base coat which is like a stain block and then paint over it but if the sanding is not flush smooth it was always be noticeable

You could line the walls with lining paper, its cheap, easy to do and will give you a great finish.
If I see walls that are not just for just painting I always line them you get the best finish.

You should have not applied Zinsser coat and it would have been better if you just filled out, and sand the wall then just re coat it.

It sounds like you've taken the right steps in preparing the walls for painting, but there are a few factors that could be contributing to the sanded areas showing up after painting:

1. **Depth of Sanding:** If you sanded the uneven areas back to the plaster, it's possible that the repaired areas are now smoother and more porous than the surrounding painted areas. This can cause differences in how the paint is absorbed and how light reflects off the surface, making the sanded areas more visible.

2. **Primer Coverage:** While Zinsser Bulls Eye 123 is a good primer, ensure that you applied it evenly and sufficiently to the sanded areas. Priming helps to seal the surface and provide a uniform base for the paint, but if the primer coverage was uneven or insufficient, it could contribute to the visibility of the sanded areas.

3. **Paint Application:** Make sure that you're applying the emulsion paint evenly over the primed surface, paying extra attention to the sanded areas. Applying thin, even coats of paint can help to minimize the appearance of differences in texture between the sanded and unsanded areas.

4. **Lighting:** The appearance of the sanded areas may be more noticeable under certain lighting conditions, such as direct sunlight or overhead lighting. Consider adjusting the lighting in the room or adding additional light sources to minimize the contrast between the sanded and unsanded areas.

5. **Touch-ups:** If the sanded areas are still visible after applying multiple coats of paint, you may need to touch up those areas with additional primer and paint to achieve a more uniform finish. Feathering the paint out beyond the sanded areas can help to blend them in with the surrounding surface.

If the sanded areas are still showing up prominently after trying these suggestions, it may be helpful to consult with a professional painter or decorator for further advice and assistance. They can assess the situation in person and provide recommendations tailored to your specific circumstances.

Because they have not been sanded properly on different areas

It sounds like you've done the right steps in terms of priming and painting, but the issue might be with the sanding technique or the type of paint used. Here are some possible reasons why sanded areas on walls might show up after painting:

1. **Insufficient Sanding:** If the raised areas or marks on the walls weren't sanded down enough to create a smooth surface, they may still be visible under the new paint. Ensure that you sand the areas thoroughly until they are completely smooth and blend seamlessly with the surrounding surface.

2. **Type of Paint:** The type of paint used can affect how well imperfections are covered. Flat or matte finishes tend to highlight surface imperfections more than satin or semi-gloss finishes. Consider using a paint with a higher sheen to help mask imperfections.

3. **Paint Application:** Make sure you're applying the paint evenly and using the appropriate technique to minimize the visibility of sanded areas. Apply thin, even coats and use a roller or brush to blend the paint seamlessly over the sanded areas.

4. **Primer Coverage:** Ensure that the primer has been applied evenly and covers the sanded areas completely before applying the topcoat of paint. In some cases, additional coats of primer may be necessary to achieve adequate coverage.

5. **Drying Time:** Allow each layer of primer and paint to dry completely before applying the next coat. Rushing the drying process can lead to uneven coverage and visible imperfections.

If you've addressed these potential issues and the sanded areas are still showing through, you may need to consider alternative methods such as skim coating or using a different type of paint. If you're unsure, consulting a professional painter or decorator for advice can be helpful.