Which grade of sandpaper to use for removing paint from plastered walls?

We've removed a lot of wallpaper in the house. Underneath is a layer of paint. A lot of this paint has come off with the wallpaper leaving bare, smooth plaster underneath. The walls and plaster are generally in a good state. There is still a lot of paint on the walls but the transistion between paint and plaster is quite obvious when you run your fingernail over it and will definitely show when painted over (I know from past experience). We are in the process of sanding the paint and edges down. Can somebody please advise on what grade of sandpaper is best to use. We've been using 80 and that has worked reasonably well at smoothing the walls and paint down. It has taken a lot of elbow grease and time so far though! Would we be better off using something rougher or finer?
TIA Marc.c
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Well I just did this in a bathroom, but I was tiling the walls and I removed the paint since it was flakey in places and would not provide a secure base. I scored the surface (not caring about the plaster under) and removed some with a steam stripper and scraper, and dry-scraped the rest. How well a stream stripper works depends on the paint, but you could try it. This may give you a lot less depth to sand. Also, are you using an orbital sander or doing it by hand ? Exactly what works best for this type of job depends totally on the nature of the paint and wall. I would use the coarsest paper that does not damage the plaster underneath, and use one of the oxide papers since these do not clog as much as the traditional stuff. Good look. Simon.
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Thanks for the reply Simon. Im using an orbital sander with 80g paper. After looking around on the internet I can now see that 80g is medium coarse. Seems to be doing a reasonable job so I think I will stick with it. Im thinking that any coarser may damage the plaster. Marc.c
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Cokey wrote:

You'll see it anyway, mainly because it's a different texture to plaster. Feathering the edges helps but it's better to get it off if you can. A heavy duty scraper works well, especially if you go over the paint lightly with a hammer to craze it
We are in the process of sanding the paint

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If you go through the finishing coat of plaster you may find that the undercoat is gritty and feels rough no matter how carefully you try to sand it. This then absorbs a lot of the moisture from wallpaper paste and makes it difficult to keep the paper stuck down. The grit also shows up under the paper as bumps and invites people to scratch...
You didn't say, but if it's gloss paint this can be a real pain once it starts to flake: can't get it off and can't get rid of the edges either if you decide to leave it! I had this probem in a bathroom: some idiot had gloss painted the walls then papered over - paper never sticks properly and soon goes mouldy - especially if this is a cold outside wall. Had a nightmare getting the paint off: which included copious caustic soda (heat just sticks paint harder to the wall) and then wash down into bath. Then of course, plaster is impregnated with mould spores... Cured by hefty dose of zinc sulphate solution on the plaster; then lining paper; then gloss paint on the lining paper. Bathroom fine for many years after that and much easier to change paper than paint!
Moral of story: don't paint straight on plaster - looks much better with lining paper first anyway.
Footnote: that zinc sulphate solution really was excellent, but I tried in recent years to get some more (I used to be a chemist) for another bathroom and could find no suppliers. The chemical now only seems to be used in veterinerary preparations... Had some lengthy debate about this on the Screwfix forum at the time.
Cheers,
S

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What concentration and amount of ZnSO4 would you recommend for tihs? Is it OK to add some to paint for mould prevention? So far I've used aspirin, which converts to salicylic acid in the paint and seems to work well.
NT
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I think I used 10% w/v, but of course, when it dries it's effectively 100%. Obviously, it depends how porous and how sealed the plaster is with other residues of paste/paint etc, as to how much it will take to make sure the whole surface is covered effectively. It would pay to do several quick thin coats letting it dry rather than run down.
At the time I used it, ZnSO4 was a very cheap chemical that was a standard item in most laboratories, so it was a very cheap and effective thing to use. It's solubility does mean that if you leave the treated surface uncovered, condensation will gradually leach it out, but if you do what I did, and cover it in paper and a couple of coats of gloss, it's there for as long as you want it to be. Also, when you use paper and gloss in this way you don't have the hassle of black lines of mouldy grout to deal with either! (The same bathroom is half tiled, and I have never been able to keep the grout free of mould - though the tiling is before my time...)
I could find no products with just the zinc sulphate in - they tend to go for complexity and very high price these days. There used to be a time when Chemist's Shops actually did sell chemicals: now it's only brands! So much for progress.
If you do manage to find a supply, do remember that though it seems pretty harmless to us, it is not something to splash around unnecessarily into the environment, where in most cases the 'bugs' and moulds are vital.
Interesting point about the aspirin. I don't know why that should affect the mould, but come to think of it I don't remember seeing many fungi on willow trees - though I have had some excellent 'Chicken of the Woods' from them on occasion!
One last point that should be pointed out just in case: some old houses being renovated may just still have some lead paint hanging around, and others may even have paint over asbestos containing materials: be careful what you lay into with those sanders, and make sure you wear a decent mask and use dust extraction if you can.
S

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Thanks Steve, useful info there.
Salicylic acid is antifungal, http://en.wikipedia.org/wiki/Salicylic_acid and aspirin which forms the acid is readily available and extremely cheap, safe, and often already to hand, so an ideal way to make paint mould resistant.
Copper compounds are help prevent spoilage.
Chemists not stocking chmicals is a pita, but they have to go where the money is. Last time I got something it was very old stock, must have sat there decades. He finally found someone to sell it to :)
NT

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Cokey wrote:

Coarse paper shifts will shift the substrate more quickly but doesn't give a good finish; fine paper will give a nice finish but won't move material in a reasonable timeframe.
So whenever you're sanding anything, it's the same basic rule: start with coarse and work down through the grades, getting finer.
David
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Yep.
Tip: get one of those foolscap or A4 sized concertina folders and put your wet and dry grades from 1400 down to maybe 100 in those, and buy the coarser grades on a roll. If you are really on the ball you can have different folders for sand/glass/aluminium oxide etc. Keeping a good set of grades like this means you rarely get stuck working harder than necessary with the wrong grade or type. Remember that a polished surface is really just one where the scratches are very small and all the same. (As opposed to a surface which has just had the scratches filled in with 'polish'.)
S

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You could always do what I did. That was say bugger that and have it skimmed :)
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