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?
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.
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.
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
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.
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
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
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.
Thanks Steve, useful info there.
Salicylic acid is antifungal,
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
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 :)
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.
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'.)
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