Sanding wall to be painted - Home Theater Forum and Systems - HomeTheaterShack.com
 
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post #1 of 7 Old 08-13-07, 01:52 PM
Jono
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Sanding wall to be painted

I picked up a quart of Behr UPW #4850 Flat Exterior and am going to use it to paint a 92"~ screen on my wall. I thought about going grey with it, but I think I'll leave it as white for now. After spending alot of time reading AVS DIY forums, the atmosphere here seems more friendly and helpful. I greatly appreciate all the rolling and painting techniques outlined here.

Now, I want to sand the wall a bit to make it as smooth as possible. I have a fine sanding sponge that can be used. Should the sanding always be done wet? The wall currently is painted with a light green latex satin paint.

Also, any tips on adjusting the projector image on the wall so it is perfectly rectangular (or as close to possible). I had it table top mounted and it was perfect, but since putting it on the ceiling last week I'm struggling to get it perfect. I've made sure it's as level as possible, but right now the bottom right of the image bends downwards slightly. After it's squared up as good as can be I'll trace the outline of the screen. The projector is the BenQ W500. Thanks
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post #2 of 7 Old 08-13-07, 02:23 PM
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Re: Sanding wall to be painted

First things first, welcome to the Shack!

You can go wet or dry.

When we say wet, you dip the sanding sponge and then squeeze the water out. You don't want it to be sopping wet. This helps keep the abrasive grit clean and free of build up. When the sanding block surface gets a paint build up it no longer is sanding evenly. What I do is if it's a small section I don't bother wetting the sponge down.

If there is a lot of texture use the rough side to knock it down quick, and then the lighter grit fine side to smooth everything out.

If your walls are really rough and uneven, you might want to look at liner paper and then put a very thin skim coating of drywall mud on and sand that smooth with the 3M sanding block.

After you get the wall smooth, put a coat of Kilz2 on and get a projector baseline.

As far as the screen image geometry, it sounds like you'll need some keystone correction. As long as it's not drastic you'll be okay. Smokey can provide more information on that though so I'd hit him up for some detailed information.

How far back is the projector mounted? Right now it looks like you're at 15fL of light on the screen so you are wide open for any screen option you want. If you are planning on viewing during the day or with stronger than normal ambient lighting, we can get you going in that direction with many solutions. If its going to be mainly for movies and light controlled viewing there are options for that as well.

Where are you located? If you're west of the Rockies check out Do-Able. The white side is one of the best white screens available and you can paint the reverse side with a gray screen for a reversible screen. If painting the wall is your choice, no problem there either! We're flexible

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post #3 of 7 Old 08-13-07, 02:43 PM
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Re: Sanding wall to be painted

Thanks for the reply and I'm glad to be here

I thought about other substrates, but I think I'm going to go with painting the wall, for now anyway. I can't find Kilz2 here in Canada, but from reading Tiddler's experiments, the Behr UPW Exterior #4580 is quite good and is self-priming so I picked up a quart of that to put right over the current paint after a bit of sanding.

I don't want to use any keystoning since with the lens shift and my placement I shouldn't need to use it at all. Maybe I'll just need to play around with the mount some more to get it adjust better. What's the easiest way to make sure the lens is perpendicular to the screen? I've tried to make the projector as level as I can making sure it's not tilted up or down or leaning left or right using a level.
 
post #4 of 7 Old 08-13-07, 03:00 PM
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Re: Sanding wall to be painted

Somebody really needs to build a ceiling mount with a built in level like the hangman cleats.

I have a very small level that's only six inches long, you could try that to make sure the projector is pefectly level which should put it perpendicular to the screen. I agree and would think your lens shift should be able to get you all lined up.

Send Smokey a PM and I'm sure he'll have the best answer for you!

"Make everything as simple as possible, but no simpler." - Albert Einstein

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post #5 of 7 Old 08-13-07, 07:14 PM
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Re: Sanding wall to be painted

Measure from one side of the wall to the center of your screen. Use that measurement at your pj, setting the center of the lens at that point - measure from the same wall along the ceiling. If it's a big drop, place a small pencil mark there and hold a level plumb from that mark to the lens. As long as your screen is placed properly, your lens is at the center mark, and your projector is level, you shouldn't have to use any keystoning. Smokester, where are ya???

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post #6 of 7 Old 08-13-07, 07:19 PM
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Re: Sanding wall to be painted

Quote:
Jono wrote: View Post
Also, any tips on adjusting the projector image on the wall so it is perfectly rectangular (or as close to possible). I had it table top mounted and it was perfect, but since putting it on the ceiling last week I'm struggling to get it perfect. I've made sure it's as level as possible, but right now the bottom right of the image bends downwards slightly.
If you're looking up at the bottom of the projector (standing underneath it), you need to rotate it slightly clockwise, so it's aiming further left on your wall. Right now, the front of your projector isn't parallel to the screen...it's at a slight angle. The issue isn't with it being level...it just needs to be rotated in-plane.

I know this only because I just went through the same exact scenario...fine on a shelf, but ceiling mounted it and the bottom right of the image bent low. That was the fix. Unfortunately, once you get it squared up, you may find that you're now aiming a few inches left of where you originally were (and where you wanted your image to be).

If you have the ability to adjust the horizontal image of the projector, that's a non-issue. Square it up by rotating it clockwise, then use the image shift to put it where you want it on the wall. If you don't have that option, hopefully your exact screen location is flexible enough that you are OK with the adjusted screen location.

And, welcome to the HTS! Mech, Bill, and Todd are great...you'll get wonderful advice from them...without the egos and attitude.
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post #7 of 7 Old 08-13-07, 07:28 PM
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Re: Sanding wall to be painted

^^^^^ What Jim said above! ^^^^^

That'll teach me to post before reading the first post in a thread!

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post #8 of 7 Old 08-13-07, 08:40 PM
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Re: Sanding wall to be painted

Hi ya. Got your PM's, thought I'd answer here.

I know this problem as I have it myself.

There is actually 4 possible issues going on, this is why it can be difficult to resolve.

Multi choice

1. Projector placement.
2. Projector rotational alignment.(different to 1.)
3. Lens distortion.
4. Screen isnt flat, square.
5. All of the above.

Tools required.
(cheap method)String line, tape measure, rule
or laser level with laser sighting(really exellent tool to get screen centralised).

First thing to check. Your wall.
Grab an assistant....now now......with your string line check how flat your wall surface is. Do this by having your assistant hold the string line against the wall following the bottom edge of your screen area. Keep the string tense as you at the other end approach the wall suface with the string.
Follow it in by looking down the string path as you get close to the surface.

If the wall is flat the string will connect when you touch the string to the wall. You will see no hollows under the string nor will it deviate along it's path.

If the wall is curved the string line will contact at the peak point then bend the string. It will be obvious.
If you see a hollow behind the string you have a pre built concave screen Maybe not desirable unless you are doing anomorphic.

Repeat process corner to corner, virtical, top edge, bottom edge.

If the wall is out you have to repair to proceed.

Because I used MDF I was able to pack behind my screen to get it flat.
In fact if the wall is badly out, using a sheet of MDF or substrate is a cheap repair.

PJ alignment. This takes careful care.
With a 4 directional laser level, find the center line of the bottom of the screen.
Rotate the laser beam up slowly, if the projector is in the correct spot the beam should hit in the middle of the lens. Simple....yuk yuk yuk.

Cheap way.
Take your string line. You actually need 2 bits now.

Make a plumb bob, weight on the end, with a point so you can accurately pinpoint things.

From the center line of the screen, drop the plumb bob from there. Where the point strikes the floor is where you start a string line. Stretch out the string line under tension so that you go past the projector. You need your luvly assistant to hold the end by the wall as you get the line square.

Square=>3,4,5 princple, with a triangle if one side is 3 and the square(90degs) side is 4 the connecting line is always 5. This can be any size as long as the arrangement is the same multibles of 3 4 and 5.
3ft 4ft and 5ft, or 9ft 12ft and 15ft, applies with metric aswell.

Now measure 3ft from the centerline, make a pencil mark on the wall.
Measure 4ft along your string line, mark the string at 4ft.
With your tape measure arc a 5 ft path from the 3 ft wall pencil point.
You then bring the string line 4ft mark and tape measure (5ft) together, tension the string line out past the projector and ta daaaa you have a square line from the screen.

Drop a plumb line down from the projector lens centerline.
If you are perfect the plumb line should touch your screen string line.

If you are off 2 things could be wrong.
1. rotation
2. PJ Squareness.

If you rotate the PJ until it lines up the projector may be still off square.
At this point you find out how good lens shift is if you have any.

This can be tricky to comprehend, remember you have a double hinge going on here. Typically the PJ is pivoting around a different center line than the lens center line. When you rotate the PJ the lens is sweeping on an arc, the lens shift is another pivot point.

The aim is to have the lens itself aligned with the screen.

It is very likey that you are off and you make need to reposition the screen or PJ, which ever is easiest.

Distance, Lens shift. This is where I need to remount my PJ.
I need to be a little further away and lower.
With consumer grade stuff compromises are made. Lens is an area where some spend fortunes just to be optically correct.

You are always best to minimise lens shift and Zoom functions on consumer grade gear. I am getting optical distortion because I am set at 3m(12ft) from the screen and have wide zoom. If I pulled my PJ back about 1m(3ft) and lowered it about (300mm)1ft I would fix the edge bending optical distortion.


And I thought I was going to write this in 5mins.....so much for lunch break

PS, keystoning is nasty, I have now seen the light, seen what it does and is really something to be avoided.
With a 720 type PJ running on NTSC or PAL 1 click of keystone won't be noticable.
But if you run HD 1 click of ketstone will damage the image to the point of dropping the HD to SD or somewhere between.

Light changes what it is doing depending if we are looking or not. Considering we only see this as a reflection of the past....what is it really doing now?
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Last edited by <^..^>Smokey Joe; 08-14-07 at 04:42 PM.
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post #9 of 7 Old 08-18-07, 10:47 AM
Jono
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Re: Sanding wall to be painted

Thanks for all the advice guys. I squared it up as good as possible and painted 2 coats of Behr UPW #4850 Exterior Flat. Wow, the screen really pops now, much brighter. It really seems brighter than all of our CRTs.

Blacks are a bit less deep now than they were on the green wall; I probably should have tinted the paint with one of Tiddler's EasyFlex formulas. I'll give it some time and if I still desire deeper blacks I'll look into tinting.

I also must say that paint is extremely easy to work with. I followed Tiddler's instructions and have zero roller marks and a nice flat white screen.
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