first coat questions..

[tuk]

first attempt as skimming.....read a few threads on here on what causes tiger stripes etc ...not sure if this is what i have ...cant really feel them by hand but can see them ..looks like the surface of the wall went off at different times(first coat showing thro?) but got flattened in(see mid up left).....wall still hasn't fully dried out yet..the lite pink makes it look better...any ideas? ..the second pic is a few days after

and was wondering for a newbie would it help giving the first coat more time to go off ..say 30 mins . ..also can someone explain the first coat finger test

 

[Capital spread]

Hi tuk

Even the best plasterers get tiger marks sometimes with the finger test the first coat should be just starting to firm up before you apply the second, knowing when this is right comes with experience.

 

[christ]

right tuk..... when doing reskims u want to put the second coat on while the first 1 is still wet.. u get tiger stripes because the second coat was really thin and was apply when the first coat had coat off to much... next time u skim a wall try it like this and u might understand better....

pva wall blah blah

mix up enough plaster for the first coat

and mix up enough for the second coat and make this mix a bit thinner so it spreads over the thicker first coat which is wet.

apply the first coat dosnt have to be to neat. also dont bother to flattern

then apply the second coat straight after, from the thinner mix. and apply as neat as u can.

then flattern the wall and leave untill it has gone chessy.

then when it is cheesy start trowling up, in an up and down motion, taking the plaster back of the wall to get it flat and so there are no blemishes. also on your first trowel dont use any water . then use a little on the second trowel. and then crosstrowel it for final trowel.

 

[beddy]

stripes can also come from using too much water if the wall is still a bit wet, only use enough H20 to make the trowel glide easier.

 

[nelly]

As capital spread says 2 posts above, it happens to everyone.


The reason for 'tiger stripes' or the 'marbleing effect' is because of varying trowel pressure/depth of plaster.


It doesnt happen on plasterboard as much as a re-skim, because plasterboard has a uniform surface but a reskim can be hollows then high points and everything in between, its a non uniform background, which is generally why you have been asked to re-skim it really .

Whats happening when you trowel plaster up is, you are compressing it, the same as useing a wacker plate on hard core.

So when you pull your trowel over the wall the plaster is feeling more pressure on the shallower parts and less on the deeper parts, because on the deeper parts there is more 'give', its more spongy lets say.

As plasterers we know it looks shlt but is actually okay.

As an aside I have recently developed a new tool, that was for something else but a massive bonus happened with it, it virtually totaly eradicates any disscolouration in the final skimmed wall.

I have to admit I dont know how it achieves it, but it does, and it does it with ease too......................which we all like

 

[tuk]

thanks for the replies all ..very helpful

It doesnt happen on plasterboard as much as a re-skim, because plasterboard has a uniform surface but a reskim can be hollows then high points and everything in between, its a non uniform background, which is generally why you have been asked to re-skim it really .

that makes sense, the bottom part of the wall was ok ..as its old perfectly flat and uniform plaster surface...in the pic from about 6" below the nail in the doorframe and up to the ceiling is where someone has used some kind of filler leaving uneven, craggy, raised surface etc...and this is where the discoloration starts

with the finger test ..if your finger leaves an imprint in the surface its ready .but if the plaster comes away with the finger leaving a point/nipple in the surface then its too wet ..is that right?

have you got a pic of this trowel your talking about nelly

 

[christ]

if u leave the first coat to go chessey before u apply the second one, u'll have to use loads of water on the wall to stop it from blistering when u trowel up and thats why u have fat on the wall.

 

[Chris W]

more pressure, less angle..
or 'keep that f'ckin trowel flat!'
for as long as you can

pick the angle of the trowel up and what you'll do is take off whatever hasnt 'set' or 'stiffened' leaving the stiffer stuff underneath..

if you do cause a stripe, whack it back on the wall just before it and trowel it back in using firm pressure and a flat trowel...

wait till you see dark spots in it (with experience you can do this just before it starts to go dark) and THEN cross trowel (or just trowel) at around 30 - 45 degrees..

common problem with newbies is they think theyre making life easier by pickin the trowel angle up cos they dont need as much pressure, all they get is stripey walls and half a bag of mix scraped off on the bucket lip..

specially on ceilings when theyve got 'armache' :-[

 

[christ]

more pressure, less angle..
or 'keep that f'ckin trowel flat!'
for as long as you can

pick the angle of the trowel up and what you'll do is take off whatever hasnt 'set' or 'stiffened' leaving the stiffer stuff underneath..

if you do cause a stripe, whack it back on the wall just before it and trowel it back in using firm pressure and a flat trowel...

wait till you see dark spots in it (with experience you can do this just before it starts to go dark) and THEN cross trowel (or just trowel) at around 30 - 45 degrees..

common problem with newbies is they think theyre making life easier by pickin the trowel angle up cos they dont need as much pressure, all they get is stripey walls and half a bag of mix scraped off on the bucket lip..

specially on ceilings when theyve got 'armache' :-[

trowel the wall up once or twice and (scrape off on the bucket lip..)
to get it flat and remove blemishes. then cross trowel (all they get is stripey walls) which is basicly flatterning the wall to close the surface in.

what u told him to do was flattern the wall a couple of times..... which will leave a good finish if the surface is straight/square and it has been layed on perfectly. if u lay on with a permshaped trowel and all u do to trowel up,,, is flattern it a few times, it'll end up uneven. and will stick out like sore thumb when painted with silk paint....