My parents live in a old CoOp. About 100 years old. There are cracks all over the walls. What paint should I use to hide them. FLAT matte semi gloss? Would adding texture to the paint help? Ill try to attach a picture or two. Every foot or two the paint has branching cracks. The second attachment seems better. Any advice on what to do? The walls are plaster. thanks
The lower the sheen - he better.
There is a series of steps that can be taken to address the cracks; one of which is spraying it with an aerosol elastomeric, the bridge over the crack with flex and won't 're-crack'. Then prime and paint it.
Otherwise, looking at the pictures; I'd really like to know how deep the cracks go. Almost looks like a surface tension issue with a previously applied paint.
As Ask4Dan mentions, it is hard to tell the dimensions of the cracks. There may well be 100 years of paint on the walls or even painted over wallpaper under 40 years worth of paint if nobody removed it. If they are surface tension cracks, a high build, high bond primer applied with a fairly heavy nap roller might fill most in if they are narrow and not deep.
Some sort of elastomeric product could indeed work too. If they are wider and deeper than we see in the photos, NuWal---a resin and new wall membrane system might be called for to help. It is not as easy to work with as the YouTube video suggests but works out well with some patience. Enlist a friend to help. Neither of these options is inexpensive.
You could think about adding some texture but it could just increase your problems.
As mentioned, you have a lot of surface issues going on and the higher the sheen of the paint, the more they will show.