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  1. #1
    Join Date
    Jun 2011
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    1

    Default Should I sand my walls before painting

    Hi all,
    We just purchased our first home. A mad rad atomic rancher built in 1955.
    I'm all set to get my new office all painted up. kinda...sorta...but not really. I wiped down every inch, top to bottom with magic eraser. The layer of dirt was astonishing. And have now wiped the walls down with a water/vinegar solution to remove the magic eraser residue.

    Thanks to some reading around the internet as well as one video here. I believe the next step is to sand the walls. But i'm just not sure. They already have a pretty good tooth to them. I think an eggshell sheen was used.

    So do I really need to sand the whole surface? How much? what grit paper?
    Or am I over thinking it and should just move on to filling cracks and nail holes?

    Thank for your help.

  2. #2
    Join Date
    Mar 2008
    Location
    nova scotia, canada
    Posts
    1,522

    Default Re: Should I sand my walls before painting

    sanding the walls will make for a better finish coat. when walls are rolled with paint there can be a rough texture created which will then become more pronounced with every layer of new paint.

    you dont have to go over board with it, all you need is a pole sander and 150 grit sandpaper or sanding screens. just do a quick few passes over the entire wall surface to smooth it down a bit. just be sure to vacuum the dust off the walls before the new paint goes up
    fire up the saw and make some dust

  3. #3
    Join Date
    Jun 2011
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    208

    Default Re: Should I sand my walls before painting

    Tamisery:

    Generally, the rule of thumb is: If your paint is semi-gloss or gloss, then you really should sand it down to roughen it before painting over it with the same kind of paint. That is, sand down semi-gloss or gloss alkyd paint before repainting with alkyd paint. Similarily, sand down semi-gloss or gloss latex paint before repainting with latex paint.

    If you're painting over an alkyd paint with a latex, then you need to either paint with an alkyd primer first, and then top coat with the latex paint, or you sand down the alkyd paint before applying the latex paint. About the only time you don't really need to sand down alkyd paint before painting over it with a latex is if it's a dead flat alkyd paint.

    If you're painting over a latex paint with an alkyd paint, then you need to sand down the latex paint if it's a semi-gloss or gloss.

    This may seem confusing, but it all makes much more sense if you just realize that alkyd resins are very much smaller than the resins in latex paints. So, alkyd paints will stick better to smoother surfaces than latex paints because their smaller resins can fit around (and grab on to) much finer surface roughness than latex paint resins can.

    So, if your paint appears to be dull looking (either it always was dull, or cleaning with the Magic Eraser made it dull), then it should be rough enough to be painted over.

    One way to check to see that it's ready for painting is to use a Q-tip to apply some of your new paint to your wall. You only need to paint a small area (two inches by two inches is plenty big enough) in each spot you test. Allow that paint to dry completely. Latex paints are dry to the touch in a half hour or so, but film formation isn't complete until the "freshly painted smell" in the room dissipates, which will typically take 3 or 4 days. Once the paint is dry, stick ordinary masking tape (not painter's masking tape) to the paint, and then pull the tape off quickly. The more paint comes off with the tape, the more something is interfering with the adhesion of the new paint to the wall. Ideally, you want the masking tape to pull off the new paint cleanly.

    An incidental benefit in doing this kind of testing is that it allows you to tell when you have complete hide of the underlying colour. That's because after your first coat of paint, you'll actually have two coats of paint in each test spot. If you observe increase colour "density" in the test spots, then you're not getting complete hide of the underlying colour. What actually happens when you don't get complete hide is that some of the incident light travels through the new paint film, reflects off the old paint, and then travels back through the new paint film back to your eye. Some of the frequencies in the incident light are absorbed by the old paint, and that affects the colour you see. Once you have complete hide of the underlying colour, then NONE of the incident light is reflecting off the old paint, in which case you won't see any difference at all between N and N+1 coats of paint. As long as you perceive any difference, you haven't completely hidden the underlying colour.

    I'd use a Q-tip to test your walls in several spots before you paint the whole office. That way you know for sure if you need to sand or not. And, that way you know if and when you get complete hide of the underlying colour.

    PS: You don't need to know the rest...

    The way paint companies test paint adhesion is with a tool that's fitted with either 6 or 11 razor blades. They apply new paint to a substrate and then use that tool to cut through the paint both vertically and horizontally, thereby creating either 25 or 100 "squares" of paint. They then stick masking tape over that crosshatch pattern, pull off the masking tape quickly, and count the number of squares that remained stuck to the substrate (rather than coming off with the tape). "Good" adhesion is considered to be anything better than 80 percent of the squares.
    Last edited by Nestor; 06-15-2011 at 03:34 AM.

  4. #4
    Join Date
    Jun 2011
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    208

    Default Re: Should I sand my walls before painting

    Tamisery:

    I thought I'd comment on your cleaning off the Magic Eraser residue with a vinegar solution...

    Google "Basotect Foam"

    It's made by the BASF company of Germany.

    That's what the Magic Erasers you used to clean your walls were made of. It was originally used to make seat cushions for airplanes because of it's very light weight. It's cleaning properties arise entirely because of the microscopic structure of the foam. So, Magic Erasers leave no "chemical residue" that needs to be cleaned off the wall prior to painting.

    Basotect is made from a very hard plastic, which aren't normally used to make foam. However, another difference is in the amount of blowing gas used. Some foams, like "extruded" polystyrene insulation (example: the blue foam insulation sold as "Roofmate") have small bubbles in the plastic. Those bubbles are small enough that they don't connect with one another, so that kind of foam in impermeable to air or water. Other foams have larger bubbles that do interconnect to some extent, making those foams permeable to air and water. Expanded polystyrene foam (the white stuff that seems to be made of "beads") is an example of that kind of foam. To understand Basotect, imagine that ALL of the bubbles in the foam intersected A LOT with their neighbor bubbles, and what you were left with was just the plastic remaining along the lines of bubble intersection. Everywhere else was just empty air. Now, if you can imagine that the bubbles were tiny (about the width of a human hair) then you've got the correct image of what Basotect foam would look like under a microscope.

    When you clean a surface with Basotect foam, the hard plastic network the foam is made of can break, leaving tiny "bristles" of hard plastic that can get into much smaller crevices that a rag, sponge or normal brush bristle can. That's why Magic Erasers clean so effectively. But, the WAY they clean is exactly the same... they're just like a brush... but with very much smaller bristles. So, a Magic Eraser leaves behind no more residue than a scrub brush does.

    (And, since the Magic Eraser was such a success, I'd be willing to bet dollars to donuts that BASF (and other chemical companies) are now trying to make that same foam out of a TOUGHER plastic so that the eraser would last longer.)

    I'd post a micrograph of Basotect foam here if I could. This web site won't allow me to do that until my post count is 10.

    I just took a look at an MSDS sheet for acetic acid. It says that the "Percent Volatiles" in acetic acid is 100 percent. That means that a puddle of acetic acid will evaporate completely without leaving any kind of residue behind. Since vinegar is just acetic acid dissolved in water, there shouldn't be any vinegar residue on your walls either. So you should be ready to paint.

    Hope this helps.
    Last edited by Nestor; 06-15-2011 at 03:47 AM.

  5. #5
    Join Date
    Aug 2007
    Location
    Portland, Oregon, formerly of Chicago
    Posts
    1,950

    Default Re: Should I sand my walls before painting

    Jeez, all Tamisery asked was if she should sand the walls!

  6. #6
    Join Date
    Jun 2011
    Posts
    208

    Default Re: Should I sand my walls before painting

    Yeah, true... ... but if you give a man a fish you feed him for a day. If you teach a man to fish you feed him for life.

    Tamisery now knows how to check to see if her walls are ready for paint, why Magic Erasers clean so well and why the vinegar rinse water solution she used won't interfere with the adhesion of her new paint.
    Last edited by Nestor; 06-15-2011 at 01:36 PM.

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