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Old Sep 15, 2006, 07:10 PM
Speedy Tree Surgeon
Tiverton,Devon,UK
Joined May 2006
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pva glue,pva release agent is there a difference

Hi all,
Could some one explain to me what pva is used for if wax stops the resin sticking and is it a thinned down version of pva glue?

pva is pva,or not?

Tim
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Last edited by avago; Sep 15, 2006 at 10:05 PM.
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Old Sep 15, 2006, 08:34 PM
Come out swinging
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Well, I can say I use PVA (polyvinyl alcohol) and wax. I wax first, then buff to a shine, then apply PVA and let it dry, then the epoxy layup. This ensures a nice glossy finish, and an easy release from the molds.


Sean
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Old Sep 15, 2006, 10:01 PM
Speedy Tree Surgeon
Tiverton,Devon,UK
Joined May 2006
680 Posts
Hi Sean,
But would thinned down pva glue be the same as pva that you would buy from composite supplier

I'm thinking that 1Lt pva glue = 2.50
and 1Lt of the genuine article from the composite supplier =10 +

I'm on a very tight budget

Tim
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Old Sep 15, 2006, 10:35 PM
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Tim,
The short answer is no. Your PVA (white glue) is polyvinyl acetate. PVA mold release is polyvinyl alcohol. They are not the same thing.

The PVA works as a secondary barrier, since the wax "layer" is generally only a few molecules thick. A good mold surface will release just fine with wax alone... but the insurance of PVA makes most people more comfortable.

You might be able to make thinned white glue work, but why bother? Nothing breaks the budget faster than having to start over from the beginning.

Good luck,
-David

P.S. If you are using polyester resin (which I strongly urge against), there is a chemical basis for using PVA (again, polyvinyl alcohol). The surface of a polyester mold will stay "active" for quite some time and the PVA helps to prevent your layup from permanently bonding to the mold.
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Old Sep 15, 2006, 10:45 PM
Speedy Tree Surgeon
Tiverton,Devon,UK
Joined May 2006
680 Posts
Thanks Dave,that what i wanted to know.

Cheers

Tim
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Old Sep 15, 2006, 11:14 PM
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What davidfee is referring to is styrene migration from the part to the mold, something that will occur when using polyester or vinylester resins as cure takes place. This will give a stick up, greif and other unwanted problems.

PVA is good for starting molds up, and sound for epoxy as well for a secondary physical barrier. I haven't had much luck getting a smooth PVA finish I am happy with - perhaps I am a perfectionist... and have had good results with pure caranuba wax.

With the comments, regarding budgets, spend the 10 pounds, and do it right - molding stuff ain't for the feint hearted! Note, the litre of PVA will last a very long time.
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Old Sep 16, 2006, 01:55 AM
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I have tried PVA but, like samotage, have had difficulty getting a nice finish. You can't brush it on because it beads-up and if you spray it you need to use lots of *very* light coats so each coat just deposits some small beads of PVA. As soon as you put to much on it fisheyes on you.

Much better to just use a good wax and polish it well (with epoxy anyway).
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Old Sep 16, 2006, 03:30 AM
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brisbane australia
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difficulty applying pva

Quote:
Originally Posted by XJet
I have tried PVA but, like samotage, have had difficulty getting a nice finish. You can't brush it on because it beads-up and if you spray it you need to use lots of *very* light coats so each coat just deposits some small beads of PVA. As soon as you put to much on it fisheyes on you.

Much better to just use a good wax and polish it well (with epoxy anyway).

I still use pva (green stuff from FGI not the blue) over wax sierra. The only time I have had problems (usually applied with fine hair brush) is when I have an incompatible wax (usually some type of silicon involved) or I am spraying too dry ( I usually thin the pva with metho or water).

Howerver as xject suggests a lot to be gained from mastering laying epoxy straight onto wax. I have friends who do this but I haven't been game - pva over wax is easy! (after lots of pain finding compatible wax and pva - long story)

cheers Jeff
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Old Sep 17, 2006, 09:25 PM
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I should point out, that earlier attepts I have had with PVA were on a K&H release wax, that did not exhibit the fish eye or beading effects. This was supposed to be of a caranuba base, however gave a strong hydrocarbon aroma that was quite unpleasant, and required the wearing of a resiprator during application!

I had also had stick up and paint related problems with this wax.

I have switched to the TR High Templ release wax, a pure carnuba wax from FGI which is brilliant, a "wax" like aroma - no need for respirator to apply, no stick up, but doesn't like the PVA... I like this wax very much, it looks like, feels like and works like a wax should, plus it buffs up really easily too

Sam.
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Old Sep 18, 2006, 01:11 PM
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For PVA application, I use cut-up pieces of T-shirt material. Just dab the t-shirt cloth in the PVA and apply a nice coat on the mold. After a minute or so wipe it again, don't apply any more PVA, juse keep wiping with even strokes.

If you PVA is beading or showing stroke marks, you have too much on there.

See the 3rd post in this thread.

http://www.rcgroups.com/forums/showthread.php?t=470466

Sean
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Old Sep 18, 2006, 02:03 PM
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Joined Nov 2004
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After having parts stick in the mold I have decided that I will accept the finish I get with PVA. We have tried several waxes that are supposed to release without PVA but I won't try it again. After all when you build a model the fiberglass parts need to be scuffed anyway. I'm satisfied with spraying the PVA into the mold, there are no streaks and while the finish is not perfect it is good enough for a scale model. If you have ever ruined a mold because of a stuck part you might come to the same conclusion!
There are some pros out there who have the experience to make parts without PVA but for us average types I would suggest it is a good idea.
Paul
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Old Sep 18, 2006, 06:18 PM
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New Orleans, LA
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Quote:
Originally Posted by ps2727
After having parts stick in the mold I have decided that I will accept the finish I get with PVA. We have tried several waxes that are supposed to release without PVA but I won't try it again. After all when you build a model the fiberglass parts need to be scuffed anyway. I'm satisfied with spraying the PVA into the mold, there are no streaks and while the finish is not perfect it is good enough for a scale model. If you have ever ruined a mold because of a stuck part you might come to the same conclusion!
There are some pros out there who have the experience to make parts without PVA but for us average types I would suggest it is a good idea.
Paul
I agree!! I practically ruined a mold by using ONLY wax... Since then Id rather trade off the time required for the pva to dry and the possible probs with surface finish quality than toss all those hours of hard work of making a mold

BTW: when I started this whole mold making thing some 5-6 yrs ago, I was told I could use white glue (elmers type stuff). I tried it and it worked quite well...

P
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Old Sep 18, 2006, 07:03 PM
Speedy Tree Surgeon
Tiverton,Devon,UK
Joined May 2006
680 Posts
I will try out the pva glue when i've got 5 mins spare,
I'm just getting into this moulding stuff,which is why i asked the question about the 2 types of pva,but i've just done my first half of a mould with just wax and it seemed to come out allright
http://www.rcgroups.com/forums/showthread.php?t=570223

Tim
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