Plastering 101

[i_am_fubar]

So, a couple of years ago, I had a go at plastering my bedroom and apart from a few dodgy patches around the corners and window sill, I was happy with the finish. Two walls were finising coat over plasterboard. One was bonding and finishing over brickwork.

Fast forward to today, and I've been re-doing the hallway. Old plaster fell off the wall/stripped down to brick work. Got PVAed and I had a stab at bonding coat one smallish section of wall. It sucked. Set too fast, didn't componsate for the cr4ppy brickwork and it's no flatter than than when I started. Thankfully, I still have 5mm minimum to fill to get it level.

I've been advied to use multi-finish for the top coats, but no way that'll fill the gap all the way with just two coats. so, question time...

1) Can more than 2 layers of multi-finish be applied?
2) Does multifinish stay workable for longer?
3) How can I increase the work time of plaster (both bonding and finishing)?
4) Is it essential for PVA to still be tacky?
5) If one coat has properly dried (and cured), does PVA need to be applied before another layer?
6) Can more than one layer of bonding be applied?
7) What is the best technique for getting into corners (especilly top left)?
8) How flat should/must the bonding coat be finished to (not smooth, still needs key)?
9) Is it possible to blend old plaster and new plaster on the same wall without a seam?
10) Providing the bonding is properly stuck to the wall, would pros (in general) have a problem working over it?
11) While a pig to get up due to fast setting time, is out of date plaster any weaker?
12) How significantly does age affect cure time of well stored plaster (in dry garage, off floor on wooden shelves)?
13) Between layers of finishing plaster, should it be polished?

Cheers guys.

Will post more questions if they appear.

Sam.

 

[Richard C]

Bonding is primarily for use on low suction backgrounds; it can be useful on dodgy walls but if the background is high suction, you have to kill the suction to a degree with a sealing coat of PVA (apply the night before), PVA again before you start & apply the Bonding in at least 2 coats. Keep the surface moist (but not overly wet) as it’s setting or it can end up looking like a dried up river bed.

I've been advied to use multi-finish for the top coats, Thankfully, I still have 5mm minimum to fill to get it level but no way that'll fill the gap all the way with just two coats.

You can fill out 5mm & more with 2 coats & certainly in one session but may give problems for the inexperienced. One way of doing it is to put another tight Bonding coat on there & then skim with Multi as soon as it sets; very useful for plastering over heavy Artex patterns.

1) Can more than 2 layers of multi-finish be applied?

Yes but if it dries out it’ll need prepping.

2) Does multifinish stay workable for longer?

No it’ll set quicker than Bonding but I suspect your having problems with suction due to other reasons. You should aim to get Multi it on the wall in around 40 minutes & no longer than 50 or you’ll find yourself struggling with enough time to level off & trowel up.

3) How can I increase the work time of plaster (both bonding and finishing)?

There are a few theories & old wives tales involving peeing in it & using mayonnaise but no, not really. The secret is to use fresh well in date plaster, clean tools & bucket, prep correctly & don’t work in heated or overly drafty rooms.

4) Is it essential for PVA to still be tacky?

It should be really but if it’s drying out as you go it’ll be OK as long as you don’t start plastering directly over PVA that’s set (overnight).

5) If one coat has properly dried (and cured), does PVA need to be applied before another layer?

Yes, two coats as usual or it’ll suck like hell.

6) Can more than one layer of bonding be applied?

Yes; ideally it should be done it 2 coats up to a total thickness of 9-12mm but you can go thicker (up to a point) if necessary.

7) What is the best technique for getting into corners (especilly top left)?

Don’t really understand why you’re having a problem with corners, I just use the trowel but I guess what your lacking in technique & practice.

8 ) How flat should/must the bonding coat be finished to (not smooth, still needs key)?

The Bonding base coat does need to be keyed (lightly scratched) but it should be flat & level. If it’s not you’ll struggle to get the finishing coats flat & level due to varying plaster thickness which will go off at different rates leading to bumps, lumps & ridges.

9) Is it possible to blend old plaster and new plaster on the same wall without a seam?

It’s always difficult to blend Multi into old plaster work. It can be done but involves skill & sometimes a little filler may be required. But you may still see it as the texture is likely to be different between new & old. It’s often far more work that filling out level & re-skimming the entire wall which will guarantee it’ll be flat & you won’t see the join.

10) Providing the bonding is properly stuck to the wall, would pros (in general) have a problem working over it?

A pro will have no problem but it’ll need prepping as it will suck like hell if it’s dry. If not prepped correctly the Multi will go off virtually as you put it on there. Ideal situation is to apply the bonding base & skim it the same day while it’s all still damp.

11) While a pig to get up due to fast setting time, is out of date plaster any weaker?

You should never use out of date plaster it’s a false economy & will give even a pro no end of problems; the bond will be weaker as a result of fast setting.

12) How significantly does age affect cure time of well stored plaster (in dry garage, off floor on wooden shelves)?

They put the use by date on the bags for a reason but storage conditions are more important than age & a damp environment can render even well in date plaster useless. The only way to tell is to mix some up & see how quickly it goes off; personally I would never use even close to date plaster.

13) Between layers of finishing plaster, should it be polished?

No & you should never over polish plaster anyway or you’ll have the devil of a job getting paint to stick to it.

 

[i_am_fubar]

Thanks for all that.

Just to further query a few points...

It should be really but if it’s drying out as you go it’ll be OK as long as you don’t start plastering directly over PVA that’s set (overnight).

Opps, I may have done that. I've not noticed any cracking (yet). Should I strip off what I have and re-do it? Only been the one small wall so far.

Don’t really understand why you’re having a problem with corners, I just use the trowel but I guess what your lacking in technique & practice.

I would guess most of my problem was having plaster that was setting as it was coming off the hawk. Extra workability should assist here, as well as more practice.

The Bonding base coat does need to be keyed (lightly scratched) but it should be flat & level. If it’s not you’ll struggle to get the finishing coats flat & level due to varying plaster thickness which will go off at different rates leading to bumps, lumps & ridges.

Best way to scratch it? I've used a board with a few nails through it before. guessing it doesn't need to be deep at all.

 

[Richard C]

Opps, I may have done that. I've not noticed any cracking (yet). Should I strip off what I have and re-do it? Only been the one small wall so far.#;

Generally you apply one prep coat of PVA, leave it to dry, usually 15 minutes or so & then apply another coat over the top, it’s this coat that you should plaster when tacky; mix ratio PVA/water = 1:3 – 1:5. Primer/sealer or bonding PVA coats (as I mentioned in my previous post) are much stronger mix 1:3 - 1:2 PVA/water are left to dry thoroughly at least overnight but preferably for 24 hours but you still apply the prep coats of PVA/water over that before you plaster; do not over brush in this case.

If the plaster is on there & doesn’t sound hollow when you tap it I would leave well alone. If you really want to push you can it try the "Selotape test”; stick a 6” length of wide Selotape on the plaster & pull off quickly if the plaster stays on the wall leave it, if it comes off on the Selotape it’s NFG.

I would guess most of my problem was having plaster that was setting as it was coming off the hawk. Extra workability should assist here, as well as more practice.

If it’s setting that quick it’s impossible to use & only good for the skip &, from what you’ve said, you also seem to be getting the preparation completely wrong. Take some time to read through the archive posts on the Plastering Forum, lots of tips & pointers in there. Also read this; http://www.building-tools.com/exams/goldtrowelbook.pdf
You need to understand that your taking on one of the most difficult trades to learn as DIY, very few will get it right or achieve acceptable results without a great deal of practice & bucket loads of tears.

Best way to scratch it? I've used a board with a few nails through it before. guessing it doesn't need to be deep at all.

Correct way to do it is with a devil float; a plastic or wooden float with the points of 3 or 4 nails just poking through the surface, doesn’t need to be deep no more than 2mm. Score the surface when it’s firm but before it’s set using a wavy line or figure 8 motion.

 

[trowelmonkey1]

hats off to you Rich for an very comprehensive reply/explanation mate!

i've been for a pint after work today and lost the will to live after question 3, unlike me i know but i've had a heck of a day, dodgy PVA me thinks.

stick with this one fubar, he'll not see you wrong.