Delay setting time of Plaster of Paris

[jrbernier]

  I am working on some scenery(first time since 1992) and want to delay the setting time for 'Plaster of Paris'.  I think I used vinegar???  If so, how much do I add to a typical '1 cup water/2 cups plaster' mix?

Jim Bernier

[cacole]

White vinegar is said to delay the setting of plaster, but how much to use probably depends on how long a delay you want.

I have used vinegar in Casting Plaster at a ratio of about 2 capfuls per cup of plaster, but I'm not sure whether the vinegar or the fact that the plaster was mixed too wet retarded the setting.

One thing I did discover is that regular latex paint will cause casting plaster to set up immediately, but the cheap acrylic craft paints don't.

[jrbernier]

Thanks!  My mind just could not remember that far back....

Jim

[bogp40]

Jim,

The colder the water, the longer working time also. High mineral content can also cause rapid setting. If you are looking for an extended open time before setting, I would try the veneering plasters (used for skim coat/ blueboard). These are Unical, Diamond and others, this plaster has the longest working time of any that I have used. Setting time can be up to 1/2 hr with cold water and low mineral content.

I use these plasters for working my final dyed second coat for scenery base.

[wm3798]

I prefer using Sculptamold, which is plaster blended with paper fiber.  It will set to a firm consistency in about 10min, but it remains damp and maleable for up to 4 or 5 hours.  It's also softer when it's fully cured, and can easily be cut with a utility knife if you need to reposition something after the fact.

 

I also like the fact that it has a little texture to it, and I tint it mixing in some grey and brown acrylic craft paints into it to keep white chips from showing up while planting trees.

You mix it up to the consistency of cottage cheese, and you can spread it on with a spatula, not unlike frosting a cake.  It doesn't run like plaster, there are no drips, and it holds its shape as you put it on.  Since it remains workable for a longer time than plaster, it's easy to press rock castings into it and make other adjustments to make a more realistic surface.  Since it's pre-tinted, I can sprinkle on the first coat of ground foam right to the wet Sculptamold, so it has a little more of a finished look before I go back an plant the trees.

The slope to the left of the bridge is the same slope above fully detailed.

Hope this helps.

Lee