Kevin's roundel attempt

[karsthotep]

That is one sexy roundel , am I allowed to say sexy? I don't know.  Anyway, Kevin that is beautiful and your former is awesome.  A little center clean up and it will be perfect.  Whats the temp down there BTW? That might account for the slower cure. 

[Theta Sigma]

Thanks to all for the kind words.  The temperature yesterday was in the upper 50's when I mixed and poured the plaster outside.  At the risk of triggering divorce proceedings, I brought it inside after it firmed up and it's been inside all night at around 68 degrees.  Even this morning, the plaster was still cold to the touch and has a wet chalky feeling.  When you rub your finger across it, it leaves a chalky residue on your finger. 

I definitely can't sand it while it's in this state.  Any thoughts?  Maybe it was bad plaster?  I never felt any heat off it at all when it was setting.

[karsthotep]

It may not actually be wet even though it feels wet, Things that are cold can give the illusion that it is wet.  Are you picking up actual moisture from it?  It may need more set time, or you may need to go with a modeling type of plaster such as the ultracal you mentioned to me earlier.  You are on the right track though,  I think its looking great.   

[atomicgraph]

Indeed what a great result you got in the end . What are you planning on using to seal it with ?

[Theta Sigma]

From what I've read, a thinned shellac product or clear gloss acrylic spray should be used to seal plaster and will harden the surface.  I haven't decided which to use yet, but the plaster has to be completely dry before you can seal it.  That can take days normally, and who knows how long with this chalky mess I've got.

An update on the plaster situation:  It's definitely not the plaster I bought.  I mixed up a test batch and left it in the bucket and it cured just fine and got slightly warm.  The surface is hard and not chalky.  

I'm starting to suspect that maybe I rushed the job and used too much plaster at once.  Also, I was having to spray some water mist on the surface to keep the finish smooth and lubricate the former.  This too may have affected the set and made the surface layer weak and chalky.  I've seen the water spray technique used in making in-situ plaster cornice molding, but maybe I got carried away and used too much.

The roundel is slowly getting better, but I'm not sure it's ever going to get hard enough to sand smooth or seal.  Time will tell.

I'm going to try again, this time with smaller thinner plaster batches so the former doesn't have to move so much excess around.  

This video which inspired me to create the roundel this way.  http://www.videojug.com/film/how-to-plaster-moulding

The technique of using a former is called a "running mold" in this situation.

Also, I may try using a smooth greased surface for the center of the roundel so that the plaster won't stick to it and I won't have to worry about sanding that area smooth.  Maybe some hardboard wall panel.  Hopefully, the flat part of the former won't create marks in the surface as long as there is sufficient lubrication.  

I wasn't expecting to get perfection on the first attempt with this, and it turned out better than I expected.  A few more tries should yield something that is worthy of sealing and making a silicone mold of.  I plan to preserve each iteration of the roundel for comparison until I find the best approach.

The beauty of this is that plaster of Paris is fairly cheap, and I can try different techniques until I find one that works.  By the time I'm done, I'll be a plaster artisan and probably divorced.  

[galacticprobe]

By the time I'm done, I'll be a plaster artisan...

Here's to hoping that this prophecy comes true, and your roundel is the finest to be found outside of a professional props shop.

...and probably divorced.  

And here's to hoping that this one never happens!

Dino.

[Volpone]

Forgive me if you've explained this upthread and for making unsolicited suggestions, but even if this roundel doesn't firm up, couldn't you use it to make a negative cast?  I'm trying to remember the sculpture class I took so long ago.  I think we used Future floor wax as a release agent but for some reason Turtle Wax car wax also comes to mind.  I don't see any underscores so you could just cast your prototype and it would probably drop right out.  Worst case, you'd have to break it out (in which case the odd set of it would actually be an asset).  Or does a negative mold not help you?  Do you need a positive to use as a form? 

[DoctorWho8]

Have you thought about baking the plaster in an oven on low heat for several hours?
Bill "the Doctor" Rudloff

[Theta Sigma]

Forgive me if you've explained this upthread and for making unsolicited suggestions, but even if this roundel doesn't firm up, couldn't you use it to make a negative cast?  I'm trying to remember the sculpture class I took so long ago.  I think we used Future floor wax as a release agent but for some reason Turtle Wax car wax also comes to mind.  I don't see any underscores so you could just cast your prototype and it would probably drop right out.  Worst case, you'd have to break it out (in which case the odd set of it would actually be an asset).  Or does a negative mold not help you?  Do you need a positive to use as a form? 

All suggestions are welcome!

There are some areas that are very rough that need to be smoothed out before any casts are made.  I've taken come close ups of this.



However, you might be on to something with the wax.  These areas could possibly be filled in with turtle wax or something of that nature and smoothed out by hand.  It's certainly an option. 

Eventually, what we want is a silicone rubber negative mold that can be used to make fiberglass roundels using chopped mat and resin.

[Theta Sigma]

Have you thought about baking the plaster in an oven on low heat for several hours?
Bill "the Doctor" Rudloff

Remember that divorce thing I mentioned earlier?   

Actually, the oven thought occurred, but I think heating it would probably crack the plaster or dry it out too much.  I suspect that the more this thing dries, the more it will start to deteriorate on it's own and get dusty.  The plaster set was just ruined by the excess water I sprayed on and I don't think there is much hope for it beyond maybe putting another skim coat over the top of what I have now.

Not to worry, I'll make another attempt soon on a new board.  In the meantime, this one can sit and dry out if it ever decides to do so.

[Theta Sigma]

Also, I don't have any silicone rubber at the moment to make a mold.  The stuff is somewhat expensive and I haven't decided what I'm going to use yet.  Leaning in the direction of "Mold Max® STROKE" to minimize the amount of rubber actually needed.  After 4 layers of silicone are applied, I would provide a base reinforcement by pouring plaster over it and using keys to make sure it realigns properly when removed from the plaster base.

http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1135/index.html

[karsthotep]

Ive used smooth-on OOMO products with good results, I think they have solid products and very helpful instructional video's too.   Your approach is what I was going to do as well. 

[Theta Sigma]

Anyone care to take a guess at how much silicone rubber would be needed to create a mold?  Would two trial sizes of Smooth-on product suffice, or would a gallon size be required?  The gallon sizes are super expensive.

[atomicgraph]

i think i sealed my dalek dome plaster plug with pva mould release. the problem i ran into was that as soon as i kept adding layers to the plaster the top layer of plaster would almost instantly drying and getting clumpy. i guess the problem i had was that the bottom layers weren't cured completely. adding the pva mould release was like magic it fromed a film over the surface and i was able to smooth it all out with just another layer of plaster. i still had to do some sanding afterwards though. i gotta say, your roundel is looking great can't wait to see how it all turns out.

[Theta Sigma]

Well, after a week the plaster on the V1 roundel was dry as a bone.  Unfortunately, it's not really of much use due to it's cosmetic flaws, but at least I know to just leave the plaster alone for a week.  Patience is not one of my virtues.

Here is the V2 roundel, this time with a wood center instead of plaster.  Some filler had to be used, but the center definitely came out better.  This one has been sealed with shellac, which is why it's glossy.  This is the best result yet, but I'm still not satisfied with it.




I'm going to stock up on some patience, and try a V3 roundel and I may tweak the former a bit.  This time I'll leave it alone for a week before I do anything else.