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I'm looking into repainting a classroom in an NGO facility (can seat about 80). Due to both time and budget constraints, I don't have freedom of spending too much time priming and painting the entire room. I suspect I can readily prime half the wall height and paint the whole room within the allocated time.

Would I create an odd mark on the wall if I only prime up to about head height? I plan to use a dry roller between the primer and bare walls to even out the edge so it doesn't get a sharp bump.

The rational for priming half the room is because the marks on the walls are all chest height and lower. I've already used a power sander and lightly sanded all the marked areas, and have sanded all repaired sections of the wall. I'll want to put primer on the repaired areas and the marked up areas to help with paint adhesion.

2020-09-01 Update: Wall material mixed. I can tell there is concrete and dry wall, with a layer of wall putty. Region is Hong Kong, which is typical to simply do Putty + Surface paint, skipping the primer. I know this doesn't work well and high humidity = putty failing, and paint peeling off.

Sanding wise, it is light sanding. It isn't past the paint layer. It is mainly to get the surface scuff marks off so the primer can attach better to the existing paint (instead of some random material). Any deep gouges are filled with wall putty and sanded flat.

If I had more time, I would sand past the paint and putty layer, and do a thicker sealer/primer layer onto the material. The current paint is not showing any major area failure, so I think it'll hold a repaint.

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Just to answer a rather odd question the way it was written...

No if you plan on going to specific height and stop right there all the way around. You will create a paint seam and it might take a few coats to make it less noticeable.

Yes if you kind of go to a height and taper off randomly on both side of the height with less paint/pressure as you go up.

My advice would be to cut corners, in the corners (and to get more people to help). You can prime pretty quickly - and paint - open walls. If you are not worrying about the ceiling or edges painting is twice as fast. I would suggest you prime to ceiling but start tapering off as you go up and the top 6-8" need very little.

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If you only prime half the wall, there is a good chance there will end up being some visible flashing between the primed and un-primed areas when you put up the paint. Primer is going to change the interface between the paint and the existing wall surface, but if that is only done for part of the wall, then you get 2 distinct regions.

How obvious it will be is going to depend a good deal on what is currently on the wall. If it is at all porous (like bare sheetrock or un-primed joint compound) it's going to be really bad. If it's an oil paint or a high quality latex, it might be more tolerable. How visible it will be also depends on lighting angle and intensity. Brighter light and sharper angles will intensify flashing like that. Higher gloss levels will also work against you.

You did say that some areas got hit with a sander. Did you put any joint compound up to even out the surface? Did you sand down to the the paper ( or burn all the way through it) anywhere? If so, those spots are guaranteed to stick out if not primed, preferably two coats.

One option you may not have considered is to skip the primer and put the money/effort into putting up multiple coats of a top shelf Sherwin Williams or Benjamin Moore paint. They are not cheap (but if you catch them on sale they are not much more than the junk from Menards) but the covering and ease of applying is dramatically better versus the cheap stuff. Many are advertised as self-priming, which I normally don't buy into, but if you need to cut some corners it might work out.

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