Fellyo wanted to suggest the method he has been using, which is an X-Acto mitre box and razor saw to trim blocks with. He comments that this method works very well and that when he sands blocks, he can rarely get them perfectly flat and it takes forever. Using the razor saw the blocks come out far straighter and require less touch-up work afterwards.
I use two or three different methods, depending on how much material I want to remove from the cast pieces and what the pieces have been cast in (which type of plaster).
For stronger plasters or smaller areas I tend to use a foam sanding block, which can be picked up reasonably cheaply from your local DIY/hardware store. The other good thing with these are the fact they can be roughly shaped and are good for getting into awkward areas.
For softer plasters, larger areas or misshaped excess materials, I tend to use an
X-Acto knife with either a chisel or short, 45° angled blade. Care must be taken when using sharp tools, so if you can, affix the block in a vise before you shave the excess plaster off.
For removing large areas or excess plaster that is harder I tend to use an electrical hobby sander or a Dremel tool. My desktop version is made by Cool Tool, and is the Unimat One machine. They can be ordered from
stockists, a list of which can be found on their website: http://www.thecooltool.com.
The handheld version I use is made by Mini Craft, which is a UK handheld hobby power tool.
Warwizardpaktu found that
when he tried adding sand to his plaster the resulting blocks were very
hard to sand, shape or cut. He ruined a fair amount of saws trying
to trim his pieces. So, if any of your plaster's contain sand or any
related silicon dioxide (SiO2) additive, you may have problems cutting it.
Fellyo tends to use an X-Acto razor saw and miter box but adds that
mixing your plaster thinner helps when you need to trim or shape your
casts. He also wanted to pass on a hint learnt from CC Crow.
Mr Crow does all of his casting in Hydrocal and finds that softening the
piece with a few drops of water before cutting helps. The website
for reference is: http://www.cccrow.com
BMT Gander added if your trying to cut any dental plaster (or
Excalibur), you should ideally do it in the first 48 hours. After that,
it's quicker to just cast additional blocks to trim rather than attempt to
try trimming the hard blocks.
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