# Forum Home Renovation Plastering  Plaster compounds whats the diff?

## Mitch1500

Can anyone tell me what the difference between the various compounds are?
eg whats the diff between base coat and jointing compound? Where does multi purpose compound come into it? And top coat vs spakfilla vs multi purpose jointing compound. 
Firstly I watched a heap of DIY videos, read a heap of articles etc on how to patch a large hole, eg. 750 x 600 mm
In bunnings they suggested I could use multi purpose jointing compound to do the lot. 
So I cut out the hole evenly, boarded it behind for support, then added a new piece of plaster in the hole which left about a 5 mm gap, I then filled the hole with the multi purpose joint filler (boscote) and sanded it back.  
Next I added the fiberglass tape and covered it with more multi purpose, covering the tape.
Next I did another coat (same compound) spreading it out further.
It seemed a bit difficult to sand so I got some UNI-PRO Smooth coat (skim coat) and added a final coat. It also seems thick to apply and sand. 
When I sanded it back I got a lot of bubble holes, so I covered the whole lot with spakfilla.  It looked ok until I sanded it. 
The end result was the I could see and feel like a map texture in the plaster around the edges etc, (Island on water) I guess this Is due to one compound being softer than the other, as sanding doesn't make it better and only seems to take out the spakfilla. It seems flat apart from the fine map edges. I've been invaded by Europe  :Smilie:  (exaggerating) 
How can I avoid and fix this? More Uni-Pro smooth coat? And asked in the title whats the diff between the compounds and which are best to do the stages? Product suggestions even? 
Thanks in advance

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## Rod Dyson

Here we go. 
All plaster based compounds (powder form) are setting plaster, that is, they set by a chemical reaction when mixed with water. Typically these are harder when set and dificult to sand and should be used as a base coat. They are best scraped back when set but still "wet"/"green"  
All the other premixed compounds are drying compounds, that is, they air dry and are typically used as top coats over the base coat. They are all softer than base coats therefore are much easier to sand. 
Some premixed products dry harder than others and have different shrinkage rates, to allow the product to be used as an all purpose compound to be used as both base coat and top coat. The main downside to these products is the drying time requried between coats. They need to be perfectly dry before applying the next coat. This is compounded when you are filling deep cracks or holes, it takes much longer to dry. 
If you combine these products you will find, (as you have) that whilst they are all sandable the sand at a different rate, thereby causing and uneven result. 
To fix what you have done you need to use one product only and re-coat the entire area and sand again.  Next time just use the top coat to repair the bubbles. Even us  pros get bubbles in the top coat over a painted surface.  Rather than trying to sand them back it is better to scrape the tops off them and give the area a very very thin skim of top coat that just fills the bubbles. It drys in 1/2 an hour and is ready to be sanded.  The problem sanding these bubbles, is that, the bubbles get filled with dust and appear to be gone.  then when you paint the paint lifts the dust out of the tiny holes and you end up with little black pin prick holes in the painted finish.  If you get this after the firest coat of paint just repair as mentioned above. 
Just as a sanding tip for others, once you have sanded back to the base coat you need to stop sanding. By continuing you are only making a problem worse by sanding all the top coat from around the offending base coat, leaving you with an uneven finish. If it requires more sanding because the base coat is raised, I suggest scraping it back in that area then apply another top coat. Or you can simply apply another top coat spreading it wider each side, then give it a sand. 
Cheers Rod 
P.S NEVER use a drying compound under tiled areas.

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## Mitch1500

Thanks that's a handy bit of information. 
All the products I used were premixed. I needed to see how thick they were before I started mixing my own. 
I think maybe I had too many raised ares In the base coat. Maybe I'm applying it too thick or not leveling it out well. 
I need to sand in between coats as this is the first time I've had a go at it, so you can imagine how rough it is. The more I try to feather it out the further I get  :Smilie:   
Green Plaster I guess you mean you can still see moisture in it but cant press your finger in? This when the scraping off is done? Or earlier? 
With the skim coat should I dilute the premixed top coat with water? 
I've got a lot more patches to do yet 
Cheers

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## Rod Dyson

> Green Plaster I guess you mean you can still see moisture in it but cant press your finger in? This when the scraping off is done?

  Yes that is right.   

> With the skim coat should I dilute the premixed top coat with water?

  You can, but usually you don't need to. 
Cheers Rod

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## Mitch1500

I watched your video again, and you make it look easy  :Smilie:  
But now that Ive applied another top coat it came up so well I may not even need to sand it. Especially now I know about the scraping off method. 
But I will have to fill in a few holes still. 
A couple of questions.. 
If I'm using fiberglass tape should I try to cover it with the first base coat so it can't be seen or should I see an imprint of the tape? 
When I was doing the top coat I was feathering it out from top to bottom and then side to side, Sometimes it would rip a small section out of the middle Like a sunken Island, and other time I would end up with small holes from match head size to a dried pea size
In the end I got it pretty good but what causes this? 
I had to try and press them out..

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## Rod Dyson

You only need to cover the tape on the first coat, I don't need to tell you what I think of fibreglass tape though  :Smilie:  
You need to flatten you trowell out a bit on the final sweeps.  This will get rid of most. 
If you are taping standard joins onto raw plasterboard you should not get any bubbles. if you do it is most likely you are applying the top coat too thick.  On a painted surface even if it is prepped well, you will get bubbles no matter how well you apply the top coat.  The reasons for this is you don't get the same absorbtion as the plasterboard.  
Cheers Rod

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## SlowMick

Rod, 
can you clear up something for me - can you tile over premixed wet area compound?  i assumed I could but maybe not if it is a drying compound. 
the stuff is like trying to spread chewing gum on the wall.  would buying a bag of normal basecoat be a better bet?  i was going to buy a bag to do the ceiling so i can just grab a bigger bag. 
Cheers, 
Mick

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## Rod Dyson

Yes you can tile over the wet area compound as it dries like and acrylic type product.  The reason you cant tile over pre mixed top coat or all purpose compounds is that theyare a clay based product that returns to is original form when it get wet.  So if you have dampness under your tiles the top coat will go back to mud and the tiles will pop off. 
Base coat stays hard even when damp and won't let go.  
Cheers Rod

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## SlowMick

Thank you very much for the prompt reply.  the bathroom may be ready for christmas after all.

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