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Hi Guys, I was puttiong some finishing plaster on top of old bonding
plaster. Previous guy had tiles straight on top of bondiong, tiles came off very easy

Now the wall is as rough as can be so I used a thick coat of finishing plaster but then noticed that you were only suppose to use it as a skim

Whats wrong with using a thicker coat anyway (5 - 10 mm). This was 2 weeks ago and it seems fairly firm

Also, I was plastering another wall and the whole bucket went off literally in 15 mins (different bag). Bugger to clear out of the bucket and scrape off the walls. Why did this happen? Only bought the bag a week before

wow...'how not to decorate'

1) bonding is extremely porous and shouldnt be tiled onto. Tiles are porous too, yes they ARE.. most of em anyway. Thats why tile adhesive is waterproof. Recommended backgrounds to tile onto are - render (sand and cement doesnt deteriorate if it gets damp), tile backer board (hardibacker etc, for the same reason)
chances are there was too much suction from the bonding when it was tiled anyway which is another reason the tiles will have fallen off dead easy.

2) you can apply finish plaster thicker than the recommended thickness and as long as it dries through it'll be fine. How you managed to get it flat at 10mm thickness is beyond me though... ill bet its ripple city in there...

3) tiles are heavy, there is a limit to how much weight a skimmed finish will take before the bond gives up, putting it on thick only reduces this limit.

4) finish plaster is porous too.

5) lots of reasons plaster goes off in the bucket. Bad bag, shelf life of about 3 months did you check the expiry date? hot water makes it go off quick. contaminated / dirty water will make it go off silly quick, if youve got a patch to do try knocking up a small amount using the water from the wetbucket... you'll have about 15minutes and then youve had it.

if youre happy with the flatness of your wall then i'd advise a coat of external pva at 4:1 water:pva followed by another one letting them both dry completely followed by a coat of thistle bond it or wickes plaster bonding agent to seal the wall completely against water ingress and give you a key for the tiles

if youre going to pva the wall dont use cement based tile adhesive. If youre going to use cement based then use sbr instead of pva (styrene butadiene rubber, any builders merchant)

chrisw w wrote:wow...'how not to decorate'

Well at least they were easy to take off

chrisw w wrote: Thats why tile adhesive is waterproof.

Didn't know that,?

chrisw w wrote: 2) you can apply finish plaster thicker than the recommended thickness and as long as it dries through it'll be fine. How you managed to get it flat at 10mm thickness is beyond me though... ill bet its ripple city in there...

I can't plaster for anything but area is small and a quick 17 hours with a sander may help

chrisw w wrote: 3) tiles are heavy, there is a limit to how much weight a skimmed finish will take before the bond gives up, putting it on thick only reduces this limit.

Tiles are thin (6mm) and light so should be ok me thinks, only small area (kitchen splash back area

chrisw w wrote: 5) lots of reasons plaster goes off in the bucket. Bad bag, shelf life of about 3 months did you check the expiry date? hot water makes it go off quick. contaminated / dirty water will make it go off silly quick, if youve got a patch to do try knocking up a small amount using the water from the wetbucket... you'll have about 15minutes and then youve had it.

how long should plaster remain workable?, Not before expiry date

chrisw w wrote: if youre happy with the flatness of your wall then i'd advise a coat of external pva at 4:1 water:pva followed by another one letting them both dry completely followed by a coat of thistle bond it or wickes plaster bonding agent to seal the wall completely against water ingress and give you a key for the tiles

if youre going to pva the wall dont use cement based tile adhesive. If youre going to use cement based then use sbr instead of pva (styrene butadiene rubber, any builders merchant)

Its a cement based adhesive (I think - Mapei S1 flex)
Stupid qustion time, I have a load of WP1 left over can I use that on top coat of SBR, then SBR, then tile or am I being a simpleton ?

Thanks Chris

yup, for an unbelievably strong bond for tiles, soak em in a bucket of water before you stick em up...
weight should be fine.. its only when you get into 10mm natural stone tiles you start pushing it...
workability time is down to a number of things- ambient temp/humidity (freezing cold will make it hang around for hours, silly hot and you better be quick), suction levels (experience, not something i could describe properly but if you give it a lick of pva first and it dries almost immediately theres plenty suction), how much you play with it (playing with it whilst its on the wall and still damp, i.e. flattening it before its ready will only draw the water to the surface and it will set quicker)
and...
how thin you mix it... knock it up like cream and it will come off the trowel thin, therefore theres more water than there is plaster and it will go off sharpish...
knock up like cream cheese and it will come off the trowel thick and youll end up with the standard chemical set time as per suction levels, temp etc...
rule of thumb - lay on, 20mins from start you should be flattening off...
knock up a fresh mix, repeat same again...
2nd mix would be around 1/2 the amount of first coat...
20 mins from start, flatten off..
trowel up, trowel up, final trowel, clean up...
a half decent spread can get two 4x2.4 walls on in 20mins easy...
its all in the timing, the experience of suction control, the speed and neatness of application and the skill in the hand...
not to mention the knowledge in the head..

errr... whats wp1? do you mean wba? if so yes, fine use it once the sbr is dry....
only prob with pva and cement based adhesives is they react, most cement based stuff is bagged... the readymixed stuff is usually fine..
if its premixed and only a splashback, a couple of good coats of pva and tile it...
if its perfection your after with perfectly flat, waterproof, super strong shower tiles.... use the sbr/wba method.. and a cement based adhesive.. or render it..

Thanks Chris

I meant this stuff:
http://www.trades-direct.co.uk/modules/ ... e=2038-102

I tanked my bathroom and have half a tube left.

I ran out of the Bal APD but have SBR so will use that

just had a look...
that wp1 is ideal... forget sbr and wba, that will do both jobs perfectly..
proper job

cool, bought way too much of it, expensive stuff too!