Lost foam coating?

Discussion in 'Lost foam casting' started by Mark's castings, Nov 17, 2018.

  1. I see that people coat their foam patterns with drywall mud, which looks like a plasterboard jointing compound/cornice cement??. I'm at a bit of a loss for a comparable Australian product apart from jointing compound so I'm going to try a mix of plaster of paris with water and PVA glue for added flexibility and thinning to the desired consistency. So far it has a short pot life of ten minutes before thickening up but seems to have reasonable strength when cured. I'm not sure how how well it will let gas escape during a pour. I'll have to wait and see how it goes.

    wheel guard patched.jpg
    wheel guards coated.jpg
     
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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Will this be aluminum casting? If so, many things will work. Plaster should be fine but as you mention, it does set quickly. Not sure about the need for PVA. Keep it thin and dry it well.

    Yes, I use the non-setting joint compound for gypsum wall board. It's the finishing compound that is used on top of taped joints not the cement used under the tape. It never sets, it just air dries so once you thin a batch, you can store it indefinitely in a sealed container. I would say about any fine powdered art clays that can be slip cast would work well too. FishbonzWV dug some clay out of a mound in his backyard, ground it in mortar and pestle before mixing/thinning, and it worked well.

    If the coating is to be used with higher melt temp metals like bronze and iron, you may need to use higher refractory coating materials.

    Best,
    Kelly
     
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  3. Mister ED

    Mister ED Silver

    Drywall = Gypsum Board = Plasterboard = Sheetrock ... joint compound = drywall mud.
     
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  4. Mister ED

    Mister ED Silver

    Kelly, are you using a premixed and thinning it down? I obviously got the wrong stuff (powder). When I went to put on my second coat, I opened my bucket only ti find a solid block, LOL.
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, premixed joint compound. Nothing scientific about my choice. It was a half full 5 gallon bucket I had/have that's fifteen years old and I'm still using it. Some of these so-called top coats have vinyl in them and I wouldn't think that would be desirable. I thin it just to the point it begins to pour from a cup. You can thin it more for dipping but takes longer to dry and may produce some cracking or crazing if very thin.

    Best,
    Kelly
     
  6. Thanks for clearing up the confusion guys, yes it will be for aluminium castings. I've been taking a look at some of the data sheets for the Gyprock plasterboard company and a lot of their jointing products have plaster of paris as the main ingredient with minor additions of chalk and lime. I used about 2% PVA glue and it doesn't seem to have added much to the overall properties, it's still a hard friable material when dry/cured. I do have some high temp flint clay too, I'll try using some of that mixed as a slurry. Plaster seems reasonably strong/hard but as always venting will be the main issue.
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I experimented with a few coatings including a concoction that was mostly mullite and kaolin. All produced the same surface finish which was good but limited by the finish on the foam pattern not the coating. However, not all wetted and brushed/applied as easily as the drywall compound. In the end, I couldn't improve on the drywall compound and it was essentially free so I focused on other things.

    Just keep the coating thin and it will breath fine. There is no improvement to be had from thicker coating. Just enough to cover and turn the pattern white is all that is required.

    Best,
    Kelly
     
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  8. Jimmy Cogg

    Jimmy Cogg Silver

    I would be very interested to see the result
     

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