Sealing Plaster

[haywire]

What is a good way to seal plaster for use in molds?

I tried Killz but that failed. Not only did it not seal the plaster, it made it softer and brittle.

I tried vaselin thinned down with alcohol, but that also failed.

Suggestions?

[Scott Hasty]

What is a good way to seal plaster for use in molds?

I tried Killz but that failed. Not only did it not seal the plaster, it made it softer and brittle.

I tried vaselin thinned down with alcohol, but that also failed.

Suggestions?

Green soap, a floor paste wax compound, petroleum jelly or liquid dishwashing liquid.

Okay, that answers that question, but I really need more information before you use any of those (or I tell you how to make the first two). Is the plaster your positive or negetive? What material are you using with the plaster to create your cast? What kind of plaster are you using?

In all honesty, plaster is a poor moldmaking substance unless you are working with an absorbsive mold process. Plus, the plaster you get at your local home center is of very poor quality. The cheapest stuff I use is Hydrocal at about $30 for 100 pounds.

Scottie

[Kylwell]

I would have tried Varathane. Good heavy plastic sealer. Let it cure a coupla days.

Scott's right tho'. You'd be better off with Rock Hard Water Putty.

[haywire]

I tried the Durham water putty and the same thing happened. My mold material bonded with the putty through the Killz seal.

The plaster was plaster of paris.

[Andrew Gorman]

I have never done this myself, but several craft books have recommended sealing plaster with shellac. It can be thinned down as much as you want, and besides, they squeeze it out of bugs!
Andrew

[Lt. Z0mBe]

I know very little about casting, but could plaster be sealed with CA glue?

Z0mBe

[Scott Hasty]

I know very little about casting, but could plaster be sealed with CA glue?

Z0mBe

Unknown, but I wouldn't try it. Plaster works in molds, because of its absorbsive qualities. It may just absorb the CA before it could create a barrier.

Again, if you have to go to the trouble of radical experimenting, then it's probably not the right method to begin with.

Scottie

[chas b]

I've been using a method I read in an article by Dave Sisson ( very talented Brit scratchbuilder). It works for me!
Use a good plaster of paris.Once dried spray it with gloss auto spray from a can.Once dry wax it several times.

Hope this helps.
Chas.

[Shinnentai]

I have never done this myself, but several craft books have recommended sealing plaster with shellac. It can be thinned down as much as you want, and besides, they squeeze it out of bugs!
Andrew

I've done this, it works fantasticly! It's not so much a barrier coat though, as much as something you do to eliminate the porousness of the plaster (or water putty) before applying the wax or petroleum jelly. Also toughens up the plaster's surface a wee bit.

The gloss auto sprayChas B reccomends above may be the same idea, just with different chemistry.

[starmanmm]

Ok, this is along the same subject. I picked up some of those molds that you make rock formations and the guy at the store told me that the hydrocal (that the molds recommends) does not work well and to us plaster of paris. Well, I did and either the mix had too much water or the humidity in the air is keep the stuff from drying out.

So my question is, has anyone used plaster of paris for these types of forms and if so can I just prime it with anything before I paint it?

Also, if I am suppose to us hydrocal, where do you get 100 lbs for $30?

[Shinnentai]

Regular hardware stores won't usually carry it, check under "building supply" or "construction supply" in the yellow pages. Here in San Diego the go-to vendor is Squire's Belt Material Co. downtown.

The guy you spoke to at the store doesn't know what he's talking about. Denser plasters like hydrocal and ultracal work WAY WAY better than plaster of paris. They're more durable, shrink less, and have much shorter, much more reliable cure rates.

Plaster of paris sucks for casting. The only thing it's good for is carving or sculpting w/ plaster as the primary media because of it's softness and relatively low weight. As you've noted, it's very humidity sensative. I live three miles from the beach, and POP never fully dries/cures when I use it, no matter how it's mixed. Durham's Rock-Hard water putty is way better. Pity I can't buy it in 100lLb bags for cheap.

[starmanmm]

So, if I understand... Durham's Rock-Hard water putty is better to use than Hydrocal? I can get both, not in very great quanities... so either of these products would work then.

[Scott Hasty]

So, if I understand... Durham's Rock-Hard water putty is better to use than Hydrocal? I can get both, not in very great quanities... so either of these products would work then.

No, it isn't. I've used it, but never as a mold. I've only used it in final castings and even then it's only when I need something quick and dirty.

Scottie

[Shinnentai]

Like Scott says, Durhams is definately NOT better than hydrocal, I just meant it's better than plaster of paris. If the job only calls for plaster of paris, use Durham's instead, but if the job call for hydrocal (or ultracal) Durham's just flat out won't cut it.

I use it (Durham's) mostly for little quick and dirty mother molds, and for plaster sculpture. It's a little tougher than plaster of paris, and has much more reliable drying/curing. For permanent or precise mother molds, stick with hydrocal. Durham's expands slightly as it cures instead of shrinking, which can sometimes lead to the mother mold cavity being a tiny bit smaller than the rubber mold it was built around. Sometimes this just means a tight fit, sometimes it means the rubber just doesn't fit at all, depending on the size and shape of the mold.

This expansion can be an asset when casting with Durhams, as it can be calculated to to compensate for any inevitable shrinkadge in the mold material, however this potential benefit is outweighed by the fact that Durham's turns out soft & easaly marred when it's mixed to a properly pourabe consitancy. It's intended as a putty, so it is designed to give best results when mixed to a thick sludgy/muddy consistancy generally unsuitable for casting from anything other than low detail open-faced molds.

Since the amount of expansion is preportional to the abount of water in the mix (and its strength is inversely preportional), it's best to mix it THICK when making mother molds. Basicly keep adding powder to the mix until it's like a loose water clay, and use it like a heavy thixatropic.

Fantastic stuff for plaster sculpture though. Applies, Carves, shapes, tools, and sands better than plaster of paris. Makes for a much tougher, more detailed piece.

Sorry; got carried away. That's way more info than needed, especially since it's not reccomended in this context.

[JimPV]

I have never done this myself, but several craft books have recommended sealing plaster with shellac. It can be thinned down as much as you want...

I've done this, it works fantasticly! It's not so much a barrier coat though, as much as something you do to eliminate the porousness of the plaster (or water putty) before applying the wax or petroleum jelly. Also toughens up the plaster's surface a wee bit.

I've used thinned shellac on plaster too, with great results. You can brush it on, and as soon as you notice it's not absorbing it anymore, stop. Or if the part is small enough, you can soak it in a thinned shellac. Spraying on works good, too. Definitely toughens up the surface and makes it more chip resistant. Plus, of course paint, won't be absorbed into the plaster.

[Warped Speedster]

Another good alternative to POP is "Quik-Fix." It's made by "Custom Building Products" and is available at most Home Depots. It mixes and applies just like plaster but it's a lot stronger and won't shrink near as much. You can drill it and screw it or glue it. Follow the instructions when using it. Don't mix it too thin, it becomes weaker if you do and will begin to shrink like plaster.

I've coated it with CA which turns the surface rock hard. I haven't used it for molds yet but it's great for sculpting or building up mountains. I've used it a lot for filling the inner leg sections of large vinyl dinosaurs (with an added rod) to retain their shape and strengthen them for mounting.