Is PVA, er PVA?

Discussion in 'Materials' started by Steve Cronje, Jan 10, 2020.

  1. Steve Cronje
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    Steve Cronje Junior Member

    Hello new friends

    I am new to composites (and this forum). I was not able to find this issue addressed, so hopefully it is not a duplicate, as it seems rather basic.

    The way I understand it, PVA is used as a mold release, and also to obtain a cure of the final exposed layer of laminating resin. I read that folks spray or brush or rub on PVA to molds.

    Can regular "Home Depot PVA" from the paint aisle work for either of these applications? I notice the inside of my FG catamaran is coated with a grey paint-like substance that mightcould be PVA, so that is what got me wondering?

    I am about to start a fairly large project (replacement bimini to go over the helm) and this is all about researching that job.

    Thanks
    Steve
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    There is a PVA, Polyvinyl Acetate, a wood glue, non-waterproof, and another PVA, Polyvinyl Alcohol, the release agent.
     
  3. Blueknarr
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    Blueknarr Senior Member

    Welcome to the forum.

    Short answer-
    NO. NO . NO

    The are not the same stuff.

    Home depot sells a PVA primer so the top coat paint sticks better.

    Mould realise PVA prevents good adhesion so the part will come out of the mould.
     
  4. Steve Cronje
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    Steve Cronje Junior Member

    Ya'know... Shucks, I missed that acetate vs alcohol. Thanks.
     
  5. Steve Cronje
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    Steve Cronje Junior Member

    Yes, thanks, I get it now.

    I have seen folks saying to apply the mould release PVA with a rag to prevent brush streaks. Is this effective? I do not have the ability to spray it for my application.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You should experiment to see what works best, on a similar surface, you may have to apply very thin coats and wait till it dries and bit, then go again, it will bead and streak and get messy if you try to brush too much on at once.
     
  7. Steve Cronje
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    Steve Cronje Junior Member

    Thanks Mr. E, I didn't realize one could apply more than one coat of PVA. For some or other reason I figured it would remove the first coat. I never tried it out. On the other hand, is there any reason to have anything other than a very thin coat? I suppose it would protect against too vigorous gelcoat application.
    I must say, the times I have brushed it on, I didn't seem to be able to get a thin coating at all.
    I tried a regular Home Depot spray mister, and that went about as well as shooting popcorn through a straw.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe if you thinned it, it may have sprayed better. Whichever way you apply it, trying to get coverage with one pass probably won't give a good result.
     
  9. Steve Cronje
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    Steve Cronje Junior Member

    Hmmm, OK. What should I thin it with? I found this on the Internets (where they also say multiple coats are the way to go, for spraying):


    Thanks for your interest.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I would think temperature and humidity would play a big part in results, but if you practice on a surface of similar porosity, you could try thinning with a little water, or a 50/5o mix of methylated spirit and water. I think the "metho" is called 'denatured alcohol" in some parts. Only when satisfied with your test panel proceed to the work piece.
     
  11. Steve Cronje
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    Steve Cronje Junior Member

    Thanks, I'll give that a try. It's denatured alcohol in North America. Is it violet-coloured in Oz? In South Africa it is a violet colour to discourage drinking of the stuff. Can't imagine.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Yep, it isn't "white lady" any more, a name that came from it being used by vagrants etc. I remember a cab driver telling me that a shop opposite a park where the homeless and derelict gathered, would have it in the fridge, but maybe that was a myth, I thin k the idea these days is to have some additive that leads to instant nausea and vomiting.
     
  13. ondarvr
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    ondarvr Senior Member

    You can thin it with water for spraying.

    You can also apply it as is with a rag. Do a small test area of 6”x6” or so. Apply enough to wet an area of the rag, wipe it on the surface, you should see the thin film of PVA on the surface, wipe until it starts to dry. With a very small amount of practice you will be able to get a smooth thin film on the mold.

    This thin film of PVA is just an insurance policy that covers any poorly waxed areas.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    The thing about PVA is that only a micro layer is required, it really isn't like painting, where a certain thickness is needed for uniform appearance, anything that actually covers the entirety, to the thinnest degree, is enough.
     

  15. Steve Cronje
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    Steve Cronje Junior Member

    In SA, it was called The Blue Train back in the day, after a famous train by the same name. Apparently desperate folks would pour the stuff into a loaf of bread in an attempt to remove the bitter, violet adulterant, then end up eating the bread anyway. Pretty sad.
     
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