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Plaster of Paris - Hints & Tips - Reference Area. - Your Model Railway Club
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 Posted: Mon Jan 26th, 2015 09:33 pm
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g0ibi
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As a life long user of Plaster of Paris, (I'm a dental technician!) I noted in a thread I read yesterday about the time plaster takes to set.
To speed up the setting time add salt (tablespoon) to the water before mixing and use room temp. water.
(To slow it down, add vinegar!)

Hope this is of help to someone in the future! :)

Cheers
Ron



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 Posted: Mon Jan 26th, 2015 10:08 pm
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mattc6911
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Good tip there thanks Ron, on a similar vein  being a Tiler by trade I always end up with part bags of tile grout. I've used this in the past instead of plaster to cast a 4ft length of retaining wall I needed on my old layout...I added some PVA to the water before mixing the grout (slightly more runny than I would normally mix for grouting)..this was poured into a mould made out of plasticine held in an old biscuit tin lid...worked surprisingly well...the mould was pressed using a plastic. Sandbag bunker wall piece from a childs wargame type set. And looked not bad at all.  The bonus with grout is you can get it in different colours. White , brown , cement grey, dark grey , limestone and many other colours. So more natural colouring and less chance of chips showing through....always worth a look in the tile section of your local diy outlet or ask in your local tile shop, For damaged bags as these can often be had quite cheap

If I can find a chunk left from the old ripped out layout I'll post a picture look

Just to çlarify ..it was cast in aprox 6" lengths with buttress pieces cast to fit between each section.......Not one  4ft length. 

:mutley 


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Matt

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 Posted: Mon Jan 26th, 2015 11:09 pm
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Chubber
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Thanks, good tip!
D



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 Posted: Mon Jan 26th, 2015 11:23 pm
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Dorsetmike
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When I use polyfilla or plaster I mix in emulsion paint to colour it (I buy the small "match pots") this also helps stop chips showing as Matt suggests with coloured grouts.



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 Posted: Tue Jan 27th, 2015 10:47 am
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gastwo
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As well as match pots, I use cheap tubes of acrylic, and those pots of ready mixed kids water-colour paints (pound shops, The Works, Tesco etc.)
Just need to be very thoroughly mixed...

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 Posted: Tue Jan 27th, 2015 12:22 pm
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Marty
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... and I use the dry oxide powders designed for colouring grout or cement here in Australia. The brand is Diggers and it's available from Bunnings.

It's very concentrated and one container goes a long way.

Good tip with the salt and the vinegar.... and of course, always mix in a clean bucket/bowl/old ice cream container that doesn't have any dried residue from the previous batch left in it.

Marty



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 Posted: Tue Jan 27th, 2015 07:59 pm
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Ed
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Thanks for the tip Ron.

It's too cold at the moment, but when I can finally get out in the garage to continue the embankment I'd like to try a bit of vinegar to slow down the setting.

Is there any specific ratio water to vinegar, or is it a case of normal water to plaster mix and add a bit of vinegar?


 Ed






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 Posted: Tue Jan 27th, 2015 08:13 pm
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g0ibi
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Just add a little to the water before adding the plaster Ed :)



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 Posted: Tue Jan 27th, 2015 08:18 pm
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Ed
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Got it, thanks again.

Ed



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 Posted: Tue Jan 27th, 2015 11:41 pm
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mattc6911
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Hi marty B.A.L used to make those colour sachets for grout but stopped making them years ago as being un economical,  never seen any one else over here doing them since

Managed to find a couple of pieces I made years ago from a plastasine mould marked out with an old plastic sandbag gun emplacement wall from a childs  toy soldier game  these pieces were only trials and i never finnished colouring or weathering them as i changed the size of the mould to suit the area it was for, but they show what can be done very cheaply if you have a large area to do . they are only about 6mm thick and surprisingly robust ( these pieces have been thrown around in a box with other stuff for the last 12 years )  these were just plain white grout and a couple have had a first wash of coulour but no more , as i mentioned you could use one of the coloured grouts being closer to your intended finnish coulour












train is just to give an idea of scale  (oo)

Cheers


   Matt


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 Posted: Wed Jan 28th, 2015 12:23 pm
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How did you stop it sticking in the p'cene mould?

My only experiments with PoP (mould a heradic device above a gatehouse) were a disaster, not encouraged by dropping the bag on the kitchen floor (my, doesn't it go a long way.
.)

Wht ratio of powder to water works best?

Doug



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 Posted: Wed Jan 28th, 2015 02:21 pm
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Petermac
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I'd always thought plaster of paris was very brittle and set far to quickly to allow for "handling" .............:hmm

For the few "castings" I've done - the odd rock mould and a bit of "Linka" stuff, I use what's sold here in France as "modelling plaster".  It's specifically designed for moulding the ornate cornices around ceilings found on "older" properties.

Sold either in the DIY outlets or builder's merchants (depending on quantity required), it sets up reasonably quickly and is much tougher than P of P.

I'm sure there's a similar product availalbe in UK without having to resort to the massively overpriced "casting plasters" sold specifically for the job by the craft outlets.



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 Posted: Wed Jan 28th, 2015 04:12 pm
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mattc6911
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Hiya Doof. Talcom powder mate...a light dusting...you know like when your putting on those tight leather trousers... Stops em sticking, but don't be too heavy handed.. Same as you do with flour to stop pastry sticking


:mutley

As for the ratio.  Its hard to say , for grouting you mix it at various ratios depending on whether you want a stiff mix to go in a wider joint or a loose mix to grout mosaics...so best bet is to just experiment, you want a slightly sloppy mix so that you can "tap it down ". To settle the mix fully in the mould and remove air bubbles..but not too runny that it has no strength, so get yourself any small plastic containers you can preferably with a bit of texture in the bottom, mix your grout, artex, ceiling coving plaster, P of P etc and pour it in..let it set and then turn it out and see what sort of definition you get, good texture, no air bubbles etc.  Just do very small batches until your getting the results you want and you will come up with your best mix. Then you can start using your proper mould..the reason for using these types of plaster / grout is because the are a finer powder, mould to texture very well and have quite good strength, those I've shown are only about 8-10mm thick and surprisingly robust. The pieces I cast to make up a 4ft retaining wall were stuck to a plywood backing..unfortunately it was built in to the baseboard and was destroyed when the whole thing had to be removed from the loft

Hope this helps !

Cheers

    Matt

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 Posted: Wed Jan 28th, 2015 04:24 pm
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mattc6911
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Oh and if you want to do a jaggy rocky hill or rock face ?  Use floor grout its a rougher texture ,mix it really loose pour it into a Polly Bag and tie the top go out to the gravel drive ,lay the bag down and gently press in down into the gravel ...or earth ..or onto a piece of bark ...whatever texture your wanting..leave to dry then peel off the Polly bag..you can scoop out a hollow in soil or sand for a hillside and line it with gravel / stone chips etc, if your wanting a jaggy rocky hilltop etc , A hacksaw blade will carve any of your moulded pieces to the required shape very easily once dry. Just remember the bigger the piece the heavier it will be


Cheers

   Matt

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 Posted: Wed Jan 28th, 2015 08:43 pm
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Chubber
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Thank you, Matt, I'm learning a lot today, air brushes and now PoP etc..



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 Posted: Tue Feb 17th, 2015 03:09 am
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paul_l
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For longer sections you can also add either paper towel or J cloth strips dipped in plaster mix, to provide some binding. Actually think I've just found a use for Collie hair, and if the dogs lucky it'll be the stuff he's shedded.

Paul



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 Posted: Tue Feb 17th, 2015 03:59 am
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Silver foxx
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well I have done all those things over the years,also used finishing plaster for moulding too it works well,even had a go with plasterers adhesive that was a breeze:roll: mopre fun than buying ready made,,
:thumbs;-):cool:



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