PVA question

[Gerald]

I built a form and made several 15 gallon ice boxes. I pulled the fiberglass boxes off the waxed form with little trouble. After applying 10 coats of release wax even the first one came off the form with ease. As a test I sprayed several coats of PVA, to the thickness of 1 mm, onto the waxed ice box form and laid up another box. I had a difficult time pulling the box off the form. My only thought was that the PVA had not set up hard enough even though I let it set over night?
I am getting ready to lay up a mold on a catamaran plug and want to use PVA. That is the reason for the test and not just because I want to sabotage a functional wax covered form.
Any ideas why the PVA wouldn't let go?
Thank You
Gerald

[Jimbo1490]

PVA does not release as easily as wax, but is considered more reliable in that it is a true barrier coat; wax is not a barrier coat. Wax works by altering the 'surface energy' thereby not letting the resin 'wet' the mold surface. When wax works it releases very easily; things can literally fall out of the mold, a hull can 'pop' out. But sometimes wax does not work as it should for various reasons. One common situation is when a mold is new. Then PVA is used to supplement the wax coat just in case the wax takes a holiday so the mold or plug is not ruined.

[Gerald]

Thank you for the response. That makes sense to me. Not the first time I had used PVA. Just the first time on a mold that had a design that had a chance of sticking. I will tear the cat plug down modify it to lay up a different design so I'm sure it will more than pay to use the PVA. I have a number of plugged and faired holes in the cat plug and 180 psi seldom fails to break the most stubborn bond so I am not really very worried.
Someone once told me that they powder the PVA with industrial talcum prior to laying up????
Gerald

[waikikin]

Gerald, I've never heard of the talc on PVA, but PVA is like a layer of safety for your tooling, 1mm sounds like too thick, 3 or 4 mist coats is all I do & theres extra safety of realease in that its water soluable & parts can be "melted" out(but messy- if done right the PVA film stays on the part & then the molds readdy for some touch up of wax & shoot some new PVA). All the best from Jeff.

[tinhorn]

Just a shot in the dark - the parts may have come off the waxed mold easily because wax is slipperly. PVA isn't slippery. In fact, in my experience, it's kind of "grippy".

I read in a trade journal some years ago that some folks spray gelcoat (mixed hot) onto new molds, then blow it out after it cures. Seems this process often solves subsequent sticking problems, and if not, gelcoat is a lot easier to sand off the mold than a part. Cheaper, too.

Seems to me that a layer of power would make the part's surface even uglier than PVA does by itself.

[Gerald]

I am not sure if 1 mm is too thick? However, I did read an article put out by a supplier of PVA that a thin layer is much worse than no PVA. They recommended 1 mm. The logic has something to do with the gel coat entering into the film of thin PVA and sticking on the plug side? I couldn't find the site again but I was written in such a way that I believed them. Then again they might just want to sell more PVA?
At this point I am not sure what he was doing with the talcum? If he was using it with PVA he must have been powdering the plug prior to applying PVA. That's my guess.
Gerald

[Gerald]

I found the site that stated thin PVA was a no no.
http://winshipmodels.tripod.com/pva_mold_release.htm

[Landlubber]

Funny Gerald, I have never used PVA anywher near that thickness either, we spray it on, and it would be only 100 micron thick maximum. You would be loosing all your shape and surface condition to do any more. I would question the author on the 1mm bit.

Actually we hardly ever use the stuff anyhow, a goos mould , highly polished and waxed and polished is good for many, many jobs.

[Steve W]

Im thinking you mean 1ml,not 1mm.As others have pointed out 1mm would be awfully thick and would take more than several coats to achieve.
Steve.