Wonky wall, best method to apply scratch coat?

[jonwestuk]

I have been teaching myself to skim walls and and am making good progress. The walls that I have done thus far are flat and smooth and when painted they are looking really good. On the next wall that I am attempting I have noticed that the original plaster is not flat has two shallow 1mm-2mm dips across the 3 meters. I am therefore wondering what the best course of action would be.

I have read that applying a few vertical lines of bonding to the high points and then applying a scratch coat to bring the wall out is the best idea. However someone I know mentioned that I didn't need the vertical lines of bonding, and that I could just level off using the existing wall with my straight edge. Surely with this method the scratch coat it going to be very awkward as it will be so thin and even non existent in some areas?

What is best practice?

PS – the wall is solid, i.e. not plasterboard.

 

[gregers]

no doubt the proper spreads will comment,but if i was doing it i would just use a straight edge and run it off.

 

[NickB_99]

I have noticed that the original plaster is not flat has two shallow 1mm-2mm dips across the 3 meters. I am therefore wondering what the best course of action would be.

1-2mm over 3m does not seem way out and could probably be made up with a skim of multi.
If it was me though, as my skimming is not pro standard, I would put my straight edge on at different heights and mark with pencil where the sides of the dips are.
a thin tight coat of bonding would only be a mm or two anyway, so put some into these dips to reduce them and then rule off with straight edge. Use a bonding agent before putting it on though! (tacky pva or WBA).
Then I would be more confident that my over skim would be flatter.

May be quite a few ways to solve this. Just my suggestion.

 

[roughcaster]

I would go with what Gregers and Newbee said. Dips of 1 or 2 mm over a 3mtr span are nothing really. No wall or ceiling is ever going to be perfect. A straight edge will always show up any bump or dip, but if it's minor, and looks good to the eye, it's not really a problem. As the guys have said, if you know where the dips are, mark them, pva the whole
wall, then put on a tight coat of bonding, but make it a little thicker where the hollows are. You can then run the straight edge up the wall just to check it over as you go. Don't try to "screed" the wall off because the coat of bonding wont be thick enough, but put it on neatly, check the "problem areas" as you go, then when you're happy, leave the bonding to go off, then put on the couple of coats of multi finish.
I'm sure you know this, but aways plaster onto the pva while it's wet/tacky.

 

[skhudy]

Your lucky, a couple of mm is nothing. I'm prepping a wall with a fireplace taken out. It looked flatish from a distance but and it's uneven and curved / bowed where the breast was taken out, and and over in inch out in places vertically as well as horizontally.

If it was just 1 wall it wouldn't be that bad, but 2 other walls in the same room almost as bad. Right pig of a room and it's tested me as a DIYer, bunch of cowboys they had in 30 years ago (date on the plasterboard)

 

[jonwestuk]

Thanks all, really appreciate all the comments. I will do as suggested and fill the gaps with a very thin layer of bonding.

I have been using PVA to seal previous bonding coats that I have done, so that is no problem, however I have read many recommendations on the plasterers forums that the best way to skim bonding is to do it fairly soon after the bonding has been applied - when it is firm but still wet. Apparently as it is still wet it doesn't absorb all the water from the multi finish.

Have I read this correctly - have any of you tried this?

 

[ree]

You can do what you propose, and skim the bonding as it turns (color will change)- do walls only, dont try a first attempt on the ceiling.
Why not, if possible, do a small area and see what happens?

You might, probably will, get pimples, but dont worry, wait a little and then hard trowel over with a clean steel trowel. If you get blisters then you've learned a good lesson.

AAMOI: why dont you knock up a mix of 3 or 4 sand to one of NHL lime, and practice scratching and floating on a bare panel of brickwork? S&L is easy to hack off, and cheap to mess with.

You could establish dots and screeds to give plumb and flatness, practice ruling off, and learn far more than by simply skimming.