Skim-coating painted plasterboard?

annika_sJuly 20, 2010

Hello everyone,

I think this may be one of my first posts, if not my very first one, on Garden Web.

I've got a question on how to proceed re-painting a bedroom. The house was built in 1938, and the walls seem to be made of plasterboard...not exactly drywall, but no lath behind them either. The previous owner used masking tape to mask off 4-inch stripes and paint pink-on-pink stripes in the whole room (latex paint). Each stripe then, because of the tape, has a ridge on the edge. I have tried a belt sander and disc sander to get the ridges down so I can re-paint one solid color, but it is taking forever and also not even going that well. I'll sand till the ridge is gone and discover I've sanded all the way down to plaster in another spot right next to the stripe.

I think it may be better/easier to skim coat the entire room. But I've never done this before and am not sure if I'm about to get into an agonizing project.

What do I need to buy and do? I have a nice wide I use drywall mud? How thick - just enough to fill the lows? Is it hard to do smoothly?

Thank you all in advance.

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The paint ridges, being quite a bit harder than drywall mud, will still show up after sanding the skimcoat; they may be a lot less prominent, but will still be visible in a raking light.
I think you need better sandpaper and a sanding block. There really are superior papers that will make this task far less arduous. The 120 grit Norton paper that has a white finish and a red backing paper is your best bet.
You can also get random orbit paper that is made for feathering paint not sanding wood. Put away the belt sander.
For a ROS 100 grit will be perfect, but by hand I'd stick with 120.
A solid rubber quarter-sheet sanding block may be found useful, or wrap a half sheet around a 6" length of 1x2 pine.

    Bookmark   July 20, 2010 at 7:35PM
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Thanks Casey...yes, the ridges ARE very hard. Surprisingly so...I always thought latex paint was sort of rubbery. I have a quarter-sheet rubber block, I will try the sheets you mentioned on it...and just expect that there is no easy way out of this, and I'm going to have very muscular arms in a few days. Haha.

Will irregular 120-grit block sanding marks then show up through my new paint?

I could kick this guy in the shins for striping this 14x14 room. Argh!

    Bookmark   July 21, 2010 at 11:35AM
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You might try getting one of those carbide scrapers, annika s, one of the pull kind, probably 1.5" wide or so. Easier and less mess than sanding down those ridges. Try to figure out just the right amount of pressure to take off the ridge without gouging the surface too much.

You'll still have to do something to smooth it out afterwards, maybe hit it with that fine sandpaper at that point. Personally, I'd probably skim-coat instead, but it it a bit tricky for a beginner, so maybe sanding would be good enough. A good skim-coat would leave a more uniform surface than sanding the old paint, however.

If you want to try skim coating, I'd recommend the powder-type (mix with water) drywall mud. I think it's called E-Z sand. I prefer it because it drys faster and doesn't shrink as it drys. Use a 12" knife. It won't go on very thick, just because of the thin consistency of the material. If you don't like the finish, give it another coat when the first one sets up enough.

The best professional advice I ever got on finishing drywall is "it's all in the angle of the knife". It's not a simple as it sounds, but once you get the feel for holding the knife angle just right, getting a good finish is much easier.

    Bookmark   July 21, 2010 at 6:14PM
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Thanks for the tips, tryinbrian.

I ended up following Casey's advice before I read your reply. It went well. A small rubber block worked better than a drywall sander. It took 3 hours and I am happy with how the ridges have gone away. I'm going to spackle a few dimples, then prime the whole deal to prevent flashing...then 2 coats of nice flat paint should look really good.

Only problem with this method my arms are temporarily paralyzed. Ha.

    Bookmark   July 22, 2010 at 12:02AM
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