Thread: sanding a concrete wall before painting?

The usual way it's done is to use a flared-cup grinding wheel in a right-angle grinder. Somewhat scary (and dangerous) to use without experience, though. You also need working space for the grinder's mechanism.

What thickness do you need to reduce to make it flush? And what are the dimensions? You say 30x30ish, but 30 what? Sounds like you mean cm for the concern you have.

Joe

If you go to any hire shop and explain the problem, they will be able to hire you a concrete renovator or a hand held concrete mower both of which look like an angle grinder or orbital sander. Get a vacuum cleaner too.

CHeers
Michael

yes, sorry 30cm possibly 40cm ... I would say about 5cm at its maximum protrusion.

I hadnt even used sand paper until 6 months ago so I need the DIY beginners 01 level of tools. I am keen to learn and will try anything but cant be credited with the assumed level of knowledge most people on this site would have.

The power tool option is easiest, but you can use an old carborundum or silcone sharpening stone and rub it back with that. Not much skill required just plenty of water and a lot of rubbing with out pushing hard. Chances are if its a soft mix it would come off easily enough, but if a strong mix on the patch you will still be there the following week. You do end up with a nice flat stone at the end, for the sharpening buffs this might be a plus.

John.

what do i do with the water? so i just kind of graze the surface with the stone? no pushing? circular motion?
How long do you think it would take if soft?

If i have to hire tools, are they expensive? and do i run a risk of screwing anything up majorly? any tips for beginners?

You spalsh the water on to the surface, and/or dip the stone into the bucket, the water keeps the surface lubricated and helps the cutting process. You use a circular motion when rubbing working on the high spots, you can feel the high points wearing away by the way the stone moves over the surface.

You don't push hard but do needs to keep some pressure on the surface, thing of the pressure as a firm massage, enough to soothe muscles but not sufficient to hit a nerve and put the victim through the roof.

I have no idea about time but if this is the zero cost option I would set aside an hour and see how it goes.

For power tools contact Kennards, Coates or any of their opposition and just ask on cost of hire.

John

great! I'll give it a whirl and see how i go. it sounds the safest option to me ... then i can just keep plodding away at it til it seems flat. Whats the best way to know its flat? Do i need to do anything else to it before undercoating? thanks.

If the hump is 50mm, then you will ba wasting your time trying to do it with a stone.The quickest way would be to chip it back behind the line of the wall, and then resurface it to the correct level.

Tools

Yup. 5cm is too much to sand, or even grind, down. Looks like hammer & chisel time.

Joe

Originally Posted by Tools

If the hump is 50mm, then you will ba wasting your time trying to do it with a stone.The quickest way would be to chip it back behind the line of the wall, and then resurface it to the correct level.

Tools

I saw 5cm and assumed it had to be 5mm, if some dill has managed to bulk out the wall 50mm then hammer and chisel is the only option and start again.

John

noooo, i was hoping to avoid that! i can refill with some filler or something from bunning right? Something that will let me paint not too many days after?

5cm protrusion.....omg how can some1 fix a wall and leave that much out.If it really is 5 cm and not 5 mm, i'd be more inclined to cut it out again and start again if thats an option.

Are you sure the wall is concrete? Or is it brick with solid plaster on it?

Tools

Originally Posted by Aangelique1611

Just wondering if someone can point me in the right direction ...

I have taken next week off work to paint my place. Recently there were some plumbing problems and the plumber needed to go through the bedroom wall to access the pipes to the bathroom. The guy who filled the wall afterwards did a really bad job and now i have a protruding concrete bulge about 30x30ish maybe slightly bigger that I would like to fix prior to painting.

Are there any concrete sanders i would be able to hire (that i can be trusted with) ... a guy at a hardware store in town suggested this thing that looked like a brick (he said i could get it from a bbq store also ... not sure what it was called though ... hope someone here knows what im talking about) he suggested using the brick thing to sand it ... just sounded a little sketchy, like it would take about 1000 hours to get it flush ... anyone else able to tell me the right way to sand back concrete?

I'm suspecting someone will advise to rip it out and start again but i would prefer not to do that if possible as 1. I want to paint next week; 2. I dont know how to do it and 3. I think it might involved curing (is that the right term?) time before painting and I really need to knock it over in my annual leave break.

Any advice would be great. Thanks everyone.

Ps. loving these forums, such good advice given.

Hi

Did he use concrete or render, if it looks sandy it's render, if it's grey and hard like a foot path, it's cement

I was told to use render to fix the holes from the plumber and the electrician in the brick work - I have a hole in the bedroom about the same size as yours.

If its render, it should respond to a normal sander with course grit paper, or a metal file or scraper.

Also, are the wall plastered? I have filled the holes with render, then I will sand them, then apply a thin coat of cornice cement (plaster) then a coat of final coat plaster. Hopefully when I have finished you will never know, I had a mini servery thru to the kitchen, thanks to the electrician L.O.L

celeste