Cracking plaster or paint

[droopsnoot]

I have this sort of cracking on some of my bathroom walls.



It's a random pattern, gets a bit more dense towards the bottom of the wall. I think the cracks are in the paint, and I can't tell whether the black in them is general grime, or black mould. Given where it is in the bathroom, it's possibly mould.

My intention is to try to flat the wall down to get rid of the cracks, then re-paint with a decent bathroom paint. Does that sound like a reasonable idea? I have sanded a few small areas, but they're in sections where the cracking isn't as bad to start with.

As it's pretty difficult going when I try to sand it on a block by hand, would it be OK to use something like an electric sander on it, or is that just going to take the plaster off as well? If I was doing something on the car I'd use a DA sander or angle grinder, but I'm never trying to retain anything other than metal underneath it. If I have to do it by hand I don't have the attention span to do it in one go, though I expect that won't be an issue - sand an area, then paint an undercoat on, then do the whole wall when it's all been sanded.

I expect if I get a professional in, they'd advise stripping and re-plastering, but there isn't very good access (which a pro would probably be used to) and I keep seeing photos of the sort of job that people pay good money for. I'd rather tackle it myself if I can, also because I've got more spare time than spare money.

[Baldchap]

Orbital sanders have changeable pads so you can put a higher grit pad (higher = less abrasive) and sand away.

That looks like the paint isn't bathroom paint to me.

Even if you buy the world's cheapest orbital sander, get branded pads as the cheap ones don't stick properly.

[Cow Corner]

Have you got adequate ventilation in the room?

Yes, you can sand it back and repaint, but need to establish whether the cause was an incorrect paint choice or an excess of moisture sitting on the surface.

[droopsnoot]

Thanks for your response.

Baldchap said:

That looks like the paint isn't bathroom paint to me.

Given the history of the paint in the bathroom, it wouldn't surprise me to find that it's had a few coats of normal paint. There's evidence that something in the house has been painted in the proper stuff in the shape of a used tin, but it's good that you've said that's the sort of effect that normal paint would give. It's not a terribly well-ventilated bathroom, but it might just have been freshened up with whatever was to hand.

Baldchap said:

Even if you buy the world's cheapest orbital sander, get branded pads as the cheap ones don't stick properly.

Cheers, there is (I think) a sander in the garage so I'll have a look at what's with it, and what it takes. The ones in the box may be no use as I can't say how many years / decades they've been in the garage waiting for someone to use them. I just didn't want to attack it with a sander and then have to come on here asking how to rectify the plaster I've ruined with it.

[droopsnoot]

Cow Corner said:

Have you got adequate ventilation in the room?

Yes, you can sand it back and repaint, but need to establish whether the cause was an incorrect paint choice or an excess of moisture sitting on the surface.

No, it's rubbish. I've asked about this before on here - the issue is that the porch on the outside of the bathroom window is now UPVC double-glazed so is very well sealed, and there's no ventilation fan in the room because the exit route is quite convoluted. I think someone said that a fan would be a requirement now if the porch was being panelled in, but it's been done for decades. I have started down the route of looking at how other similar houses here do it, but another summer has passed and I've done nothing about it. I just figured I could make a start at rectifying the finish, use proper bathroom paint and buy myself a bit of time.

ETA - and of course the problem is eased at the moment because the window is always open on the first catch, and the same for the porch windows. Once it's winter, the bathroom window will be closed most of the time.

Edited by droopsnoot on Friday 23 August 13:06

[droopsnoot]

Going back to this, I've now removed all the old paint from the worst-affected wall (I scraped it off, rather than sanding it down, as it was much quicker), and painted a thin coat of emulsion on as an undercoat.

I've got a tin of "walls and ceilings" paint that claims to be good at filling in minor blemishes, so my plan was to put on a coat of that to help smooth things out, and then a coat of proper bathroom paint on top. Does that sound like a reasonable thing to do? I'm hoping to use the walls and ceilings paint as sort of a high-build primer, though I've had to fill some bits where holes have been filled in, or I slipped when scraping the paint and dug into the plaster a bit.

[dhutch]

Give it a light rub down with some 80 grit, very light skim of toupret 'polyfiller', light sand with 120grit, emulsion?

Could whack a barrier coat on first if you have damp issues.

[droopsnoot]

I think the damp has come from the atmosphere rather than the wall - the plaster underneath the paint seems fine, which is a relief. It's a pain in the backside of a wall because the toilet is about 18" from it, making the rear corner very difficult to access. I just wondered about the walls and ceiling paint as undercoat because I've got a load of it. I started off rubbing the paint down, but it turns out that the plaster surface isn't all that flat. It's not noticeable to the eye, until you start sanding it down with a block and seeing all the low spots. Filling it sounds like a big job.

[Slagathore]

Looks like 'crazing'

https://www.thedecoratorsforum.com/how-to-repair-c...

I'd clean down with mould spray as a precaution, as if it is mould, anything you do over the top can be damaged if it isn't killed properly.

Definitely want a random orbit sander for that sort of work, otherwise you might leave some swirl marks on the wall.

120+ grit probably fine for that, unless you're trying to remove the paint altogether.

[bennno]

Rub it down, then PVA it.

[droopsnoot]

Paint is all gone, as I mentioned above I started out sanding it and then managed to get an edge and scraped the rest off in flakes. Underneath it is plain plaster, and there's no sign of anything like mould on it.

I was just thinking about an easy way to take up / fill in some of the minor surface imperfections using materials that I've already got. I've filled the noticeable blemishes - where there has been holes drilled and filled in, and where the scraper might have dug into the surface - but there's a lot of tiny stuff and I think if I use the fine filler I'll be there for weeks chasing around edges. I've already done one of the walls - a lot of that was already bare plaster because some tiles had been removed - and that seems to be looking OK, but I'm not that happy with the finish, certainly if I was paying someone I'd want it to look better. That was the first wall I've ever painted, so I want to make the second one better if I can. The imperfections probably won't be noticeable if I don't go looking for them.