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Rich Stevens
post 14 Oct 2009, 11:56
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Hi all. What's the best plaster/polyfilla/pva/ mix to use for encasing ply and scribing later to indicates stone-work? (I'm building 2 viaducts and various bridges in '00').

Many thanks,

BW

Richard
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TonyDaly
post 14 Oct 2009, 12:41
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I suppose you could use anything you have handy. Just mix it with water with a little pva added. This will help to bond it together. You can also add some acrylic or a little emulsion paint to add colour to the finished job.
Also make sure that you seal the ply with a thin coat of pva or varnish which will seal it & prevent the ply from sucking the moisture from your plaster mix before it is properly dry.I have used polyfilla,patching plaster & casting plaster in my scenic work & provided you follow the simple rules you won't have a problem.


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BobB
post 14 Oct 2009, 12:42
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Years ago when I modelled in N, I used polyfiller, painted it and then scribed it afterwards to reveal the not to fresh mortar colour. In a couple of places the paint crackled where two scribed lines met - gave the impression of broken brick. In N it gave the impression of texture as well !
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poliss
post 14 Oct 2009, 12:54
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I would avoid using Plaster of Paris as it can be dangerous if it sets around your hands. Drywall plaster with a couple of spoonfulls of white glue per 1/2 pound is much better so I've been told.


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Doug
post 14 Oct 2009, 14:40
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QUOTE (poliss @ 14 Oct 2009, 13:54) *
I would avoid using Plaster of Paris as it can be dangerous if it sets around your hands. Drywall plaster with a couple of spoonfulls of white glue per 1/2 pound is much better so I've been told.

Never herd of that before. I've had it all over the place. It doesn't set much differently to other plaster - it's just more fine and goes off a bit quicker. I use loads of plaster - whatever I can get at the local DIY, but inevitably it's a different brand and formula each time. Moulding plaster seems to be the most consistent.


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poliss
post 14 Oct 2009, 14:47
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Plaster of Paris generates heat of up to 60 degrees C as it sets. A schoolgirl lost 8 fingers/thumbs after it set around her hands.
http://www.telegraph.co.uk/news/newstopics...art-lesson.html


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Doug
post 14 Oct 2009, 15:58
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"...she put her hand up to the wrist into the bucket of plaster..."

OK, that would be a problem, but no reason to avoid using Plaster of Paris. If you are an idiot, perhaps don't, then again if you are an idiot, don't do anything as you never know what may be lurking around the next corner.

Moulding resin also works on exothermic principals, but we still use that to make moulded pieces. Epoxy too gets hot....

Many of our products and tools if used incorrectly will cause injury or death. Just read the instructions, be safe and use loads of common sense. If in doubt, seek advice.


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poliss
post 14 Oct 2009, 16:18
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Why an idiot? I never knew that Plaster of Paris generated so much heat until I heard about it on the news this week. Nor did I know how fast Plaster of Paris sets till I made a mould using it. The packet came with no instructions. If you're used to Pollyfilla you'd probably expect Plaster of Paris to set as slowly, so giving you plenty of time to get your hands out. Did you know everything when you were at school?


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Doug
post 14 Oct 2009, 16:49
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Well Plaster of Paris has been used for years to make casts around hands, arms and legs.

It is logical that when you apply a plaster cast, you build it up gradually to prevent it getting too hot. I've never had a cast by the way. These days Resin casts are used and I expect they get hot too. They have to be applied with care.

I've used plenty of Plaster including Plaster of Paris powder and Plaster of Paris bandages on the layout. Sure it gets warm, but if used normally it doesn't get too hot as heat dissipates.

Read the instructions.


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Chinahand
post 14 Oct 2009, 19:55
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Plaster of Paris has similar chemical properties to cement. When mixed with water it generates heat which is why, in hot countries, we have to use iced water when mixing concrete to help reduce the heat of hydration. The temperature in the centre of a large concrete pour can get up to well over boiling point so it is protected with hessian sacking and continually sprinkled with water for the first 48 hours as, if it gets too hot, it dries out before it has properly cured which results in weak concrete.

Getting back to the subject, I'm presently casting hundreds of 'Linka' pieces with which to clad my viaduct. The mix I am using is 4 parts (by volume) P of P, 2 parts Artex powder, 1 part of a really gungy coloured acrylic paint I have mixed up, a dash of black powder paint and 2 parts water. The acrylic gives a general stone colour to the plaster and the black powder paint, which is not mixed in too much, produces streaks of dark colour to break up the uniformity of the colour. I also use a small orbital sander attached to a chopping board as a vibrating table to get any air bubbles out.

As with concrete, once the surface has 'gone off' I keep the surface wet for about 6 hours. Again, as with concrete, it reaches about 60% - 70% of it's optimum strength in the first 24 hours and is generally at 90% strength within 3 days though it doesn't reach full strength for about 28 days.



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Graham Plowman
post 14 Oct 2009, 23:43
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I built a viaduct using this technique here:

http://www.mrol.com.au/Articles/Buildings%...ns/Viaduct.aspx

This one was done using polyfiller. Later jobs such as this:

http://www.mrol.com.au/Articles/Buildings%...unnelMouth.aspx

were done with ordinary builder's plaster mixed with a little wallpaper paste for added strength.

I find that plaster is actually an incredibly good medium for representing stone, especially when water colours are used to 'stain' it.
To that end I would not recommend pre-colouring the plaster because you'll end up with a very uniform finish which you won't be able to change with water colours. Water colours must be applied as gradual washes. It is easy to darken, but you can't lighten.
I also wouldn't recommend PVA in the plaster: it affects the absorbency which means that water colours won't hold.

Dry, white plaster with water colours added in washes gives the best stone appearance in my opinion.

Graham Plowman


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Chinahand
post 15 Oct 2009, 03:55
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QUOTE (Graham Plowman @ 15 Oct 2009, 03:43) *
To that end I would not recommend pre-colouring the plaster because you'll end up with a very uniform finish which you won't be able to change with water colours.


Sorry Graham but I have to disagree with you there.

By adding paint to the mix you get a 'new' stone look which, with the application of thin washes and selective dry brushing produces a more natural look than if you start out with white plaster/stone. It also means that any minor chips do not show up as un-natural white patches but rather as exposed bits of new stone. As it would in the real world.

You will also note that I add a little dark powder paint to the mix which produces a natural looking stratification.


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john woodall
post 15 Oct 2009, 05:04
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I use casting plaster and concrete dye, brown for land black for rocks.

It is stable and if chipped it doesnt show as white

It still takes washes and static grass easily etc.

John
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ebaykal
post 15 Oct 2009, 06:29
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All stonework of Bayland is cast. Not by ordinary plaster of paris but of a brand called CEROMOFIX, brilliant for fine casts.

http://www.eberhardfaber.com/castingPowder.EBERHARDFABER

Says what it is on the page. All you need is half hour setting time and you get results like this:



I never mix colours to it. The trick with working with plaster is; as you all know, it is porous. i.e, if you apply a brush of paint on it immediately sucks in the paint and the overall look doesn't look right. What I do is I spray all the surfaces to be painted with matte varnish filling up them pores. I then apply the paint as it now gently flows into the crevices ,giving spectacular results.



Erkut


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Graham Plowman
post 15 Oct 2009, 22:48
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QUOTE (Expat @ 15 Oct 2009, 14:55) *
Sorry Graham but I have to disagree with you there.

By adding paint to the mix you get a 'new' stone look which, with the application of thin washes and selective dry brushing produces a more natural look than if you start out with white plaster/stone. It also means that any minor chips do not show up as un-natural white patches but rather as exposed bits of new stone. As it would in the real world.

You will also note that I add a little dark powder paint to the mix which produces a natural looking stratification.


We will have to agree to disagree!
I would suggest that most buildings modelled by railway modellers are quite old if they are built of stone, therefore, a 'new' look is the opposite of what is required.
Agree that dry brushing is a useful tool.

Pre-mixing paint into plaster always has the risk that it doesn't mix evenly and then you have a situation where you may have colour bands across the plaster. This results in a lot more work to hide it. Sometimes this is successful, sometimes not and results in a time-consuming re-start.

The benefit of starting with white plaster/stone is that you have total control of where you want the colour to go. You simply add coloured washes to darken to the required level. If you pre-colour the plaster, you've already jumped several 'washes' and lost control of your colour. There is a great risk of getting it too dark. I suppose practice would give one experience, but personally, I like to be in contorl from the start.
Of course, if one uses other types of paint, that's a different situation, but I believe that anything other than a water-based paint makes a model look like its been covered in emulsion paint and takes away realism of stonework.

In my opinion, the 'chip' argument is not as bigger problem as some make out.

Graham


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