What causes the shallow hollows when skimming with multi-finish?

[RomanAbramovich]

I'm diy, this is my first attempt at skimming onto plasterboard.

I've skimmed 3 small walls and one small ceiling, everything has gone well EXCEPT once the plaster has dried out there are around a dozen shallow hollows on each wall.

Is there a technique to ensure these shallow hollows don't occur? I can't actually see the hollows when I'm at the flattening and polishing stages!

Everything else about the job is good so far - no ridges/trowel marks - just very slow progress due to inexperience.

Any advice much appreciated.

 

[stevethespreader]

Using water combined with troweling when the skim is not hard enough

 

[roughcaster]

Another thing when trowelling up, make sure the trowel blade doesn't "flatten" on the wall, keep it at the correct angle.

 

[RomanAbramovich]

Ok, thanks both.

I did the following steps:

1st coat
wait
flatten
wait
2nd coat
wait
flatten
wait
1st wetted trowel
wait
2nd wetted trowel.

(I never put any water onto the skim itself, only onto the trowel)

I'll try again, with a 'clean dry trowel' stage then another 'wait' before the 1st wetted trowel - thinking about it now I guess a damp trowel could drag the very fine top surface away if the plaster is not quite hard enough and the trowel angle isn't perfect.

 

[roughcaster]

It's all about practice Roman,,, you've got the right idea. Just out of interest, any chance of a pic of the problem "hollows"?

 

[RomanAbramovich]

Yes absolutely more practice the better. After the second wet trowel step I stood back and couldn't see these hollows (or are they known in the trade as holes?), maybe because the plaster was dark from the water that's gone over it. Once it's fully dried out pink they become visible.

 

[RomanAbramovich]

...this might help other diy'ers...

Re-skimmed the small ceiling and one wall this afternoon, as there was too much making good to do - decided it won't take any longer to pva and re-skim.

Worked out a few things I've been doing wrong...

1) Excessive trowelling when applying first and second coat - the extra strokes on the wet plaster caused more imperfections (usually hollows) than they fixed!
2) Once coat 2 gone tacky, trowel it with a dry trowel - no water used at this stage.
3) When doing dry trowel on coat 2, use more force/pressure - this seems to really iron out the imperfections.
4) Have a 500 watt site light handy to get better visibility of imperfections.

After the dry trowel I waited 15/20 mins then trowelled (polished?) with the trowel wetted. Looked good so left it at that. Will give it a few days to dry pink and re-inspect, but could see straight away it's a far better finish.

 

[roughcaster]

Well done Roman. As you said, easier to re-do the whole thing, than try to patch up. Glad it turned out well for you, and you've learnt a lesson for the future.

 

[RomanAbramovich]

...pva'd and re-skimmed all previous work, much better standard now - getting the hang of it (slowly).

I have an 18sqm boarded ceiling to skim. Worked out I'm ok doing up to 5sqm, so plan is to divide the ceiling into 4 equal sections, using 3mm stop beads as dividers. Because I do this work in the evenings, I'm able to do only one section per day - other posts in the forum suggest the dividing bead is removed soon as the section is complete and the neighbouring section started asap - obviously I can't do this, I'll need to work up to dry edges the next evening - will the section joins be obvious using this method?

 

[stevethespreader]

Best method is to not use stop beads but to use scrim work out where you are going to skim to and put a line of scrim tape along the line and skim up to it let it firm up a bit then pull the scrim tape off this action cuts the skim and leaves a nice line which is flatter than what the beads would leave if you have a look at my album you will see a 100m2 ceiling I done using this method

 

[RomanAbramovich]

Best method is to not use stop beads but to use scrim work out where you are going to skim to and put a line of scrim tape along the line and skim up to it let it firm up a bit then pull the scrim tape off this action cuts the skim and leaves a nice line which is flatter than what the beads would leave if you have a look at my album you will see a 100m2 ceiling I done using this method

Thanks. I did see your recommendations to use scrim tape for this purpose on a previous post. Sounds like a great idea, my worry is that the next evening I'd need to 'feather in' onto the previous nights skim, whereas if stop bead had used I'd need to 'butt up' to the cured skim . My feathering skills are naff, so assumed I'd get a better finish using the bead option. Will be tricky either way!

 

[stevethespreader]

Same thing you butt up to either edge be it what ever way you go but in my opinion the stopbead creates an artificial level where as if you cut with scrim its the same thickness all over