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luna 07-17-2015 05:55 AM

New Plaster to Old

I have a question regarding modeling new plaster to plaster that has cured. Even when I saturate the old plaster with water, the new plaster forms a very hard layer, often with a gray skin...this makes it very difficult to shape. Does anyone know of an additive or technique to help with this?

Thank you,

Chris_Johns 07-17-2015 03:57 PM

Re: New Plaster to Old
The underlying issue is that cured plaster, even when wet is very effective at absorbing water so it will suck the water out of any fresh plaster you put on top.

I'm not a plaster expert by any means but when I was working on a large modeled plaster form was to use a mix of alpha plaster mixed with 1/3 to 1/2 of gypsum based coving adhesive. Which was ok but you do still get some variation in hardness between layers.

I found that the best technique was to build the form from the inside out so that you can do the final layer in one go and maybe do a rough finishing before you skin it with the top coat that way you aren't cutting through different layers. Alternatively get the shape you want using whatever works and then cut it back a few mm with something quite aggressive and paint a final layer on top with a brush before final finshing.

This is the piece :

luna 07-27-2015 08:01 AM

Re: New Plaster to Old
Thanks for the tip, I will try that. When the plaster is bone dry I have used drywall compound for imperfections, which works great. It is soft, though, but these plasters are for molds and not the finished piece.

luna 03-16-2016 03:50 PM

Re: New Plaster to Old
I found a solution. Make a 50,50 mix of plaster and slaked lime. Saturate the original surface, then you can brush on the new mix for small fixes or let it thicken for modeling. There are many techniques...if you need to blend or model spray water on top of the mix so it does not drag.

Andrew Werby 03-16-2016 03:54 PM

Re: New Plaster to Old
One thing you can do is add extra water to the new plaster you're applying. That makes it softer, and offsets the tendency of the underlying plaster to suck away the water it needs to set.

Andrew Werby

Tlouis 03-16-2016 06:16 PM

Re: New Plaster to Old
Here is a simple solution .
Give the old plaster a coat of white or amber shellac. When the shellac is thoroughly dried, you can apply a new layer of fresh plaster which will not penetrate the shellac.
I have used this method numbers of times and it was always successful.

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