Lite brown marks on celling after painting.

[chrisb1357]

Hi all,

I have just applied a mist coat on our hallway celling in our flat using dulux matt thinned down with about 10% water and after about 2hrs just started to notice lite brown patches showing all over the hallway celling which looks like its where its been damp before in the pass. I know the the 2 years we have been in the flat the hallway has never had a leak in the roof so not sure whats caused this. I am now worried they will show if i apply 2 more coats of matt and not sure what to do.

Chris

 

[misterhelpful]

Your best bet is to use a stain blocking paint or oil based undercoat to hide these marks/stains before applying your finishing coats of matt.

When you then paint over these areas with emulsion you will notice that they seem patchy. Don't worry, this is just 'flashing' and is due to the paint taking a little longer to dry out over the stainblock/undercoat but it will dry the same as the rest of the ceiling.

 

[TheDec]

Just one word of warning if I may, 99 times out 100 an oil based undercoat will cure your problem, Yet on the rarest of times the salts that have formed have been known to win and again reappear. However as said it is very rare for this to occur.

For complete peace of mind you could apply one coat of alkali resisting primer just make sure that be it uc or primer that you overlap the stain by a few cm.

Dec

 

[chrisb1357]

Hi,

Can you recommend the blocking paint to get. What do you think has caused this or what could it be from.

Is it ok to apply this over the matt i have allready painted without sanding down etc.

Was i correct in applying the dulux paint thinned down for the first coat as the celling a few years back had tiles on and the other year we got all the tiles and glue off down to the plaster and left it for 12 months until we painted it this weekend. WE did the same in other rooms and had not had issues in them from where the tiles and glue had been on for years so i know its not that.

Chris

 

[TheDec]

It is normaly caused by the ingress of moisture, either past or present.

Dec

 

[misterhelpful]

Zinsser Coverstain is probably the most popular choice but expensive. DIY stores sell Polycell Stain Block which usually does the job.

The probable reason is an old leak from years ago, and as you say you have just mist coated, I presume the ceiling has been skimmed. If so, then it is likely the wet plaster has caused the stain to re-appear. If not, then it is probably just the paint that has caused it to show again.

 

[TheDec]

Chris,

Tiles, glue, try and offer a step by step guide here so we can ascertain exactly what we are dealing with.

Dec

 

[chrisb1357]

OK here Go's LOL

Step 1:
Moved into flat the other year, Council had removed the the celling tiles all over the flat inc the hallway i am talking about. They left the flaky / pealing stuff on the celling which held the tiles on so when we moved in we removed all this to leave plaster cellings.

Step 2:
The hallway celling has been left like it was for over a year with just plaster while i did other rooms first and we had no leeks during this time in the hallway.

Step 3: Over the last few weeks i filled in some area's on the plaster and lightly sanded down. The plaster is not new and has been up before we even moved in.

Step 4: Today i applied the first mist coat using Dulux Matt White thinned down a little.

Step 5: A few hours later i noticed some very lite brown patches showing on the hallway celling. These are not in 1 area of the celling. There is a few over most of it which look brown ish.

Step 6: Had a cuppa

I have never applied PVA to the walls or cellings in any part of the flat as i know this is a NO NO. I did the same for 3 other rooms in the flat to the cellings and they have been fine for over a year and did not have this issue so i dont think its anything to do with the celling tiles being up before.

Chris

 

[TheDec]

Chris,

Thanks for your detailed reply, Regarding what you have done so far Seems ok to me. Chris sometimes when a water based paint is applied to a substrate that has been covered over for many years it can often reactivate old issues.

Simply coat the stained areas with an Alkali resisting primer and when dry carry on with your emulsion.

Dec

 

[chrisb1357]

Hi,

Is it worth just painting the Alkali resisting primer on the whole celling. Can this be applied with a roller like i did with the matt paint.

I dont need to sand down the matt i have applied before hand do i. Also will the matt when painted on this apply and stick ok.

Chris

 

[TheDec]

Chris,

Well you can if you wish prime the entire ceiling yet there really is no need, just cover the stained areas. Yes you can apply the primer with a roller and yes two coats of emulsion will give you a good finish.

Dec

 

[chrisb1357]

Can you get this from places like B&Q as its the nly place around here
Chris

 

[misterhelpful]

B&Q usually only stock International brand Alkali Resistant Primer and sometimes Dulux Trade. Unfortunately, they are probably the two most expensive versions, although well worth the money.

Leyland or Johnstones are just as good but usually a fair bit cheaper from a decorators merchants.

I'm not fully convinced that alkali resistant primer is the ideal product as it won't always block water stains, should it turn out that's what you have, but it won't do any harm.

 

[TheDec]

Yes it will Mr H, sometimes when skim or boards suffer water damage salts are released within the surface. I suppose you could call this a form of effloresence, an oil based uc as I have said is normaly well up to the job yet an alkali resisting primer is the ultimate cure.

Dec

 

[misterhelpful]

Well I'm only speaking from experience Dec, as water stains in my own house have failed to cover after using it.

But hey-ho, misterhelpful always seems to be wrong lately!