Speeding up filling of butt join in plasterboard

[JMB]

Have just finished building an internal wall, one side of which butts into a pre-existing external corner. I removed the metal external corner before I attached the wall, which left a 4mm deep gap running up the join. Spent a couple of days filling this gap to bring it up level with the new plasterboard wall. Today put filler and joining tape over the butt join but would like to speed up the filling process as join still needs number of coats and would like all filling done in next couple of days. Can I put skim coats on this join using cornice cement or top coat and thus do a couple of coats on join each day?

[scooter]

JMB, check out a few threads in this same forum to find rod@plasterbroker 's website.

He mentions on his site (under patching I think ) adding salt to a cornice cement mix to make it set fast. You may be able to use this method to get the filling done quickly.

Edit: here you go, check out the rest of the site as well, it's a wealth of info.


Cheers.................Sean

[JMB]

I did check out the website you mentioned but felt a bit confused about why you are meant to do most joins in plasterboard initially with base coat which has a 24 hour drying time rather than cornice cement which is much quicker to dry. Worried I may be doing the wrong thing but think I'll give thin layers of cornice cement over the join a go tomorrow

[scooter]

Not sure about 24 hour drying time, base coat can usually be overcoated in 45-60 mins. (Hence "Basecoat 45" is CSR Gyprock's name for theirs, ie. 45 mins setting time)

Rod's site refers to base coat "setting" which enables it to be scraped back & overcoated. In the taping joins page he mentions to letting base coat set but not dry out before scraping back, the 24 hour period you mentioned may be for it to completely dry out, this wait isn't required for you to get on with the next coat.


Cheers..............Sean

Cheers..............Sean

[rod@plasterbrok]

Spot on scooter.

The top coat will take some time to dry because of the thicker base coat will take longer to dry completly.

No harm though, to add a bit of heat with a fan heater. Dont put it too close though.

Cheers.

Ps. thanks for the kind words about the site scooter. I think i'll have to get busy on the other pages planned.

[JMB]

I also find the plasterbrokers web site very helpful so thanks.

<O</OIf I continue to build up layers of filler while preceeding coats are not quite dry does this mean that it will take longer for the joins to completely dry out ready for painting?<O</O<O</O

[rod@plasterbrok]

It really depends on the weather and more particularly the air flow.

Drying is helped if you open the windows and let air circulate.

When you apply a coat to a completely dry coat that particular coat will dry much quicker that if applied over "green" plaster. "green" is the term used for set but still wet plaster.

You sound like you are doing the right thing by applying multiple coats to build up a level join, which is good. Rather than trying to fill it out in one or two coats.

I think overall the job would be completed start to finish, quicker by coating over the previous green coats, rather than waiting for each coat to dry completely between coats.

Cheers

Rod

[JMB]

Rod,

I couldn't find a heater to speed up drying but did use a fan for a while which really helped. Didnt get as much filling done as I'd wanted to due to difficulties hanging the door, but slowly getting there.

Definitely on steep learning curve with plasterboard filling. What you've said so far helps a lot. Can you please explain the difference between cornice cement, base coat and top coat in terms of their ingredients?

[rod@plasterbrok]

Base coat and Cornice adhesive are both gypsum based. The exact additives they put in to make the gypsum react in different ways, is beyond my knowledege.

The Gypsum that goes into plaster board is very "raw" with few additives. Gypsum sets with a chemical reaction, additives can alter the reaction to produce various properties that differentiate the various types of bagged plaster products. The refining process will also alter the properties of each type of plaster.

The techniques used by manufactures are kept under wraps, as plasterers seek certain qualities in the products. When they get it right they don't want it coppied.

Top coat is a clay based product, that dries rather than sets. It has detergents and other additives, like ground mica, to give it a smooth texture.

Once plaster has set, the rate of water loss is not that important. If applied in very hot conditions and water loss happens before the plaster sets, the result is a powdery, weaker plaster.

Water loss too quickly in top coat, can cause shrinkage, although if applied in a thin coat, the effects of this are rarely noticed.

Most of these products, have a water retainer in them to prevent rapid water loss.

I never install decorative cornice on a very hot day. As the cornice itself absorbs moisture from the mix, coupled with evaporation, cornice adhesive can dry before setting. I have seen cornices literaly fall off the wall due to this. Using any retarder like citric acid will make this a lot worse.

Plaster that has already set and ground back to a powder acts as an accelerator, making plaster set quicker. Salt also acts as an accelerator.

The retarders are, citric acid, lemon juice or milk. Milk is not a good idea though, as I have seen staining from the fat in milk come through the join when too much was used.

Cheers

Rod

[scooter]

Thanks for that info, Rod !

Quick question, what about a multipurpose joint compound, what do you know of the makeup of that gear?

I have used CSR's multi in the past, tried to fill too much in one hit & it didn't dry sufficiently, shrank & cracked. Their info line told me to used cornice cement for the filling, which did work.

I notice Quikcote, a WA mfr, have a multipurpose mix that is purported to "do it all", including adhering cornice & topcoating. What's yer opinion of this type (not necessarily this particular one )of product?

Just curious


Cheers................Sean

[rod@plasterbrok]

We use the muliti purpose cement in the summer here in Melbourne through the automatic taping tools. But not in winter. It should only be used in conjunction with paper tape. MPC and fiberglass tape won't give you a strong join.

If applied in a thick coat it will shrink a bit. It really has to be dried out well before appliyng a another coat. A 3 coat system is a must, (although I also recommend 3 coats system with base coat as well).

All plastering jobs come up much better if you don't try to fill too much with each coat. Over filling is perhaps the biggest error by both handy men and tradesmen alike.

The exact difference in the material content I could not tell you. But it does dry harder that normal topcoat and is sandable, where base coat is not.

There are benefits to use MPC for the handyman doing small jobs, providing they have the time to wait for the coats to dry, before applying another.

I guess the main benefit with MPC is being able buy one product, reducing the cost of jobs up to about 80m2. Anything over this and you will need 2 products anyway. Cornice adhesive can also be used to tape in a joint. The only problem is that it will set quicker, cornice cement will set faster the more it you stir it or work with it. For smaller jobs where you need to buy cornice adhesive anyway, I think it makes more sense to buy a 20kg cornice adhesive and a 20kg top coat. Rather than a 20kg MPC and a 10 or 5kg cornice cement, The cost wont be much more and you are more certain not to run out and have to buy another small quantity. They really slug you for the small quantity bags.

As far as using a MPC to put up cornice, I have never come across that product and would be very wary.

Other benefits using a MPC are that there is no mixing.

Also if you stuff up the join it can be sanded between coats.

However if you scrape back base coat when it is "green" (still wet but set) you will get rid of any over fill and finish up with a flat join better than sanding the MPC. (The problem is most DIY people are not aware of scaping back, as an option)

Cheers

[JMB]

When you scrape back the base coat what tool do you use to do this?

Butt join is coming along nicely and even starting to get the hang of doing internal corners. Tomorrow is final sand then the big test - how do joins look when painted!!!! Thanks for the advice, its made a big difference to the work I've done and clients very happy with their new wall.

[rod@plasterbrok]

If the scrape back is easy I use an 8" joint knife. But if I really have to get into it I will use a 4" joint knife.

The smaller the tool you use the more you will take off in the local area. Therefore if you have a spot that has bulged out use something smaller then finish it off with a larger tool. If you use a trowel scraping can be done using the back of the trowel. Or for those harder bits use the end of the trowel.

If the join is flat or trowelled out wide enough to blend in any build up you should not see anything when painted!

Glad the job has come out good.

Cheers.

Rod

[scooter]

Thanks Rod, appreciate your time & sharing of info

With regard to the scraping with a joint knife, safe to assume that would be a metal joint knife wouldn't it? And you would be "drag-scraping" (to coin a term , I mean like a card scraper is used in woodworking ) rather than "push scraping" (ie. scraping off an adhesive label with a razor blade scraper)

Interesting about the MPC & fibreglass tape making for a weak joint. As mentioned previously I work at Bunnings & most DIYers are daunted by the paper tape so they go for the fibreglass tape & MPC.

Per your info, they would be better off with a small bag of cornice cement & a small tub of topcoat, using the cornice cement to tape the joint (bearing in mind the quick setting time & to scrape back while green) & finsihing off with the topcoat.

The cornice cement could then also be used for any cornice work (suprise ) as well as as for gluing in backblocks, filling, patching, etc - bit more verstile for future use than if they just bought a 6kg base coat for taping.


Cheers.............Sean

[rod@plasterbrok]

With regard to the scraping with a joint knife, safe to assume that would be a metal joint knife wouldn't it? And you would be "drag-scraping" (to coin a term , I mean like a card scraper is used in woodworking ) rather than "push scraping" (ie. scraping off an adhesive label with a razor blade scraper)

Yes that is right Scooter.

Using paper tape with the MPC can be trouble if they apply it to thin and it gets dry spots before the tape is embedded. Dry spots is what creates the bubbles in paper tape.

Cheers