My sister is painting her bathroom (1/2 bath; no shower) walls. She discovered much of the previous paint had started to chip off, so she's scraped and mudded all those areas. I wonder if the sheetrock was never primed originally???

Any recommendations are welcome. Thanks!

Probably not. Had the same problem. Kilz did the trick for me. Wear a respirator.

older construction used regular rock.
Newer construction use green board,more water resistant.
Hardi board and durarock is even better.
My feeling is from my house is that it will continue to flake and peal ,that is what some builders have done in the past,cheap paint and hide their crappy work.
My first bath lasted about twenty years,I ripped the shit material and shotty work out and did it correctly from the floor up.

1/2 bath with no shower doesn't need MR board and would not receive it in today's construction either.

Failure is likely age and low quality paint. You can use a primer like kilz if you want, but primer performs as the base layer when painting new board with paper and drywall mud. If you are painting over existing latex then the quality of the finish paint is the most important aspect of the project. Go with SW, Benjamin Moore or PPG. Don't buy the paint based on what is on sale.

Thanks for the replies, fellas.

I used a premium tinted primer from the dollar shelf at Home Depot about a decade ago. Still scrubs perfectly, zero issues. The wall had wallpaper previously which had pulled off from moisture and a dissatisfied wife.

Rainforest Green is a soothing shade but if I was paying full price, maybe something different ...

The bathroom ceiling, I've always used Kilz latex primer alone, no overcoat.

Glidden Gripper primer worked well for me on many difficult surfaces.

Kilz Original.

It's oil based and nasty, smelly stuff but works great.

The finish coat(s) of latex paint stick very well to this stuff.

Never like KILZ - but do like the Zinnser products both the oil base Cover Stain and the water based 1-2-3.

However, what really stops pealing is using a semi-gloss.
They seal better than the flat or matte paint.
This is commonly recommended for this problem.
Additionally, proper prep _ sand or scrape or fill is of the utmost importance.

We use lots of 1-2-3 at work fir normal jobs and gripper when we're painting things we probably shouldn't be. (greasy kitchen walls and ceilings and vanities they don't want to pay to replace)

Zinzer 1-2-3 for primer.
Two finish coats of satin or Semi-gloss applied one day apart to seal pin holes properly.

Kilz 'or' Zinzer will work fine, so long as the substrate is in good shape. In the long run your sister may need to replace the drywall all together if it continues to degrade.