Help...what's a good release agent when casting from plaster
- Thread starter Ash
- Start date
weaselflinger
Sr Member
If I understand you correctly, and you have a plaster mold and you want to cast something in it using polyester resin; them I would seal the plaster with a couple coats of spray paint and use petroleum jelly (vasilene) as the release agent... Lots of other things would work too
Chris Martin
Well-Known Member
Oops! Talcum powder definitely won't act as a release agent. It's more usually dusted on silicon rubber moulds or vulcanised rubber moulds to get the resin or white metal into the details without trapping tiny air bubbles. Also I wouldn't use vaseline/petroleum jelly either because it will inhibit the setting of the polyester (fibreglass) resin. Petroleum Jelly can be used as a release between two layers of silicon rubber.
You need to seal the plaster with any kind of thin varnish. If it's thin it'll partly soak into the plaster and reinforce the surface which you might need when you come to separate mould from moulding.
Option one: When it's absolutely dry rub the entire thing in beeswax and buff it. You might need to do this several times. Once that's done brush on a PVA release agent specially formulated for plaster moulds. It's best to spray it but brushing is OK. Once dry start your fibreglassing. If the mould is reluctant to release once the fibreglass is set you can soak it in water which dissolves the PVA.
Option two: Alternatively spray your varnished master with several light coats of silicon release agent. You need to build up an unbroken skin of Silicon release which is best left overnight for the thinners in it to evaporate. It will feel waxy in the morning and should be robust enough for you to gently brush your first coat of gel coat on and continue normally.
Finally a word of caution from experience. Despite all the precautions a high percentage of plaster moulds still get damaged due mostly to the upper surfaces of the plaster coming away from the lower particularly if fine detail is involved. Plaster moulds are only really suitable for plain simple shapes. Be prepared for the worse.
You need to seal the plaster with any kind of thin varnish. If it's thin it'll partly soak into the plaster and reinforce the surface which you might need when you come to separate mould from moulding.
Option one: When it's absolutely dry rub the entire thing in beeswax and buff it. You might need to do this several times. Once that's done brush on a PVA release agent specially formulated for plaster moulds. It's best to spray it but brushing is OK. Once dry start your fibreglassing. If the mould is reluctant to release once the fibreglass is set you can soak it in water which dissolves the PVA.
Option two: Alternatively spray your varnished master with several light coats of silicon release agent. You need to build up an unbroken skin of Silicon release which is best left overnight for the thinners in it to evaporate. It will feel waxy in the morning and should be robust enough for you to gently brush your first coat of gel coat on and continue normally.
Finally a word of caution from experience. Despite all the precautions a high percentage of plaster moulds still get damaged due mostly to the upper surfaces of the plaster coming away from the lower particularly if fine detail is involved. Plaster moulds are only really suitable for plain simple shapes. Be prepared for the worse.
Hey I coated the inside of the mold with a couple layers of Clear Coat...I was worried to spary paint it in case the paint came off with the resin and then run the sucka down with vasoline, it worked great the cast came out just like I wanted it too... and I can reuse the mold it's going great now I just have to wait for my Resin to arrive. I was making Krypronite last night and I think my resin was starting to go bad because when I was mixing the blue and yellow it was coming out a swamp green and that looked real bad so I just overloaded the resin with red and now I have a chunk of Red Kryptonite and am feel kinda evil...lol.
-Ash
-Ash
weaselflinger
Sr Member
Glad to hear it worked out.
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