Sealing master models

[Lathanm]

I am working on a fairly large project that I need to make some cast of. The base master is made of foamcore. This was great for getting the project moving rapidly but can I use it as a master and if so how should I seal it? I am thinking on just using a couple of coats of spray epoxy appliance paint. The stuff is really strong and should be non reactive. What have other used to seal paper/foam projects for casting?

[blind pig]

I have sealed the edges with painted on pva and then spray painted it.

You will probably ruin the master when removing it, so don't for get to keep a copy for your library, should you need to cast it again.

[Lane]

Make sure you seal any cracks, not just the foamcore itself. If there are any gaps between layers, at corner joints , etc the RTV will find these and flow into them. makes it a bit difficult to remove the master.

[paulson games]

Those are good ways to seal it.

Something to keep in mind:

If you are molding under pressure hollow objects are likely to implode due to differing air pressures.

You need to make sure that hollow parts are strong enough to hold up to the pressure change or have some vent holes so that the air equalize in pressure on it's own. The vents will cause the inside to fill partially or even completely with siliconbut it's the only safe way to keep it from breaking under pressure.

It'd be a bit more complicated and time intensive but you can do the inital mold without pressure. That way the hollow object won't crush. Then you cast a new master out of resin under pressure, you will likely get surface bumps aka "BBs" from the using that mold under pressure but they are easy to remove. Once you have a cleaned your master that's now in solid resin you can redo the mold under pressure without it collapsing.

[mangozac]

I too would try sealing with PVA. I suppose you could also airbrush on a gap filling primer (a la Mr Surfacer 500) if you're worried about the PVA effecting the details.

The other option for pressure moulding hollow objects is to use a syringe to fill them with resin first. Like Jon suggests though, depending on the part I would try moulding at room pressure. To do that you need to vacuum the bubbles from the silicone after mixing, then paint a coating into all of the master's details, then carefully pour the rest over the top (ensuring that you don't create any air pockets). That way you will be able to do the cats at pressure without the porcupine spikes.

I wouldn't try casting a room pressure mould at pressure and then removing the spikes/BBs like Jon suggests. Too much of a headache when there are better alternatives.

[Lathanm]

The base structure is foamcore and is also hollow. I am covering the whole piece in paper mache to seal everything up. I have been doing some additional checking and this seems to be the best way to seal the project. It also has the benefit of allowing me to skim coat it with gesso or sand-able filler. I am not pressure casting this one. The advantage for a project this size isn't there. Who has access to a chamber that can hold a 2 foot long piece. Easier to just degas the rtv and pour it carefully. I normally paint on the first layer to keep from trapping air next to the piece.

[virusoverload]

Sorry to post in an old thread but i was wondering if you can seal a master with any primer spray paint. Such as the gamesworkshop one.?

[Fleafa]

The paint will likely come off on your mould.

[mangozac]

Proper primers (that "etch" into the material) shouldn't come off on the mould. I've never had a problem priming with lacquer based acrylic primers (i.e. Tamiya Surface Primer, Mr Surfacer, etc.) and in fact find it very beneficial to prime master parts with such products. Not only does it provide a nice, smooth, uniform surface for moulding, but the uniform grey lets you get a perfect idea of how the part really looks!

[virusoverload]

well i was looking at getting mr surfacer but im in england I could only find places in the US. Any one from my little island suggest somewhere to get something?

[Fleafa]

I bought one on eBay from Hong Kong.

[Munkey Joe]

Dont know about getting Mr surfacer in UK.... BUT possibly look into self etching primers at an Auto part store. Or even someplace that does Auto body (in America MOST better auto body shops have a small store for paints and the like). This may help in your sealing.

[mangozac]

Good tip MJ. I bought my Mr Surfacer from Japan until I found stores carrying it here in Oz. Or keep an eye out for Tamiya Surfacer Primer - in fact in my experience this has a superior etching property to the Mr Surfacer anyway.

The problem with these rattle-can based primers is that they can generally a bit too crude in terms of application and end up going on too thick and obscuring details. If you have an airbrush another option is to spray the rattle-can primer into a cup, thin it with some lacquer thinner and then shoot it through the airbrush, giving much finer control.

[Magnetic Duckling]

I've never found ANY Gunze product here in the UK but the Tamiya primer surfacer is easy to find so I use this instead of Mr Primer. Same product as far as I know, more or less.
I found a couple of stores that sells Mr Surfacer in Europe when I googled/Ebayed it. It is MUCH easier to find it in pot (?) than in rattle can though.

[mangozac]

I only ever used Mr Surfacer from the pot (I've never seen it available in rattle cans other than from Japan anyway) and airbrushed it on. Note however that you need to thin it with a retarding thinner otherwise you get awful spiderweb effects. I use "Mr Colour Levelling Thinner" (which is a retarder) with it.