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You can get an EXCELLENT picture on a painted wall, if you are willing to spend a little time ensuring you do the job properly.
1. Prepare the surface with care. If working with drywall, make absolutely sure you get every simple and bump smoothed out and sanded flat. If there are -any- dips or rises in the surface, they will translate to points of distortion in the projected image that you will definitely notice. You need a perfectly flat surface to start with, trust me on this.
2. Prime the surface with a light colored primer. Do not underestimate how much benefit this can provide in terms of uniform texture and brightness.
From here, you have a choice:
Bright-white, semi-gloss.
Grey-tinted, flat.
The bright-white semi-gloss choice provides the maximum reflectivity for projectors that might be "lumens challenged" or are being stretched to their limit in terms of projection size. If you are using a lower-brightness projector (eg: 800 lumens) and a very large projected image size (eg: 100" diagonal or bigger) -- you might find that you NEED that bright-white semi-gloss surface to produce the gain needed for acceptable image brightness.
The grey-tinted, flat finish will provide maximum contrast for projectors with lower contrast ratios. You can substantially increase the perceived dynamic range and improve the black level with a LCD or first-generation DLP projector where contrast isn't the best to begin with. The downside to a grey-tinted surface is that gain is reduced, and a projector that might produce a "dark" image on a bright white screen would likely produce an abysmally dark and disappointing image on the grey surface, even if the contrast and black levels are better.
Lastly, paint a flat black border around the perimeter of your screen. The black will absorb any overscan that you might have, as well as provide a visual contrast improvement for the edges of your projected image. The black border should be approximately 2" wide, though you might prefer a smaller or wider border.
When going with the grey-tint, pick the lighest grey or silver tint you can find at the paint mixing store. If you need more contrast and have enough brightness to work with, you can always get a darker tint and paint over it later.
Apply the paint with a good quality roller, with a tight pile, and never re-use a roller or wash it with water and reuse one, as it will bunch the pile and make for an uneven finished surface.
Greg
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