PVA to finish cure

[luis]

Today a friend told me that when he wants to finish a patch of polyester resin, (final cure) he just "paint" with polyvinyl alchool (PVA) when is done and this makes that the resin stops beeing tacky and after 24 hours he removes the PVA skin and is ready for priming. I know that he already made lots of repairs on his boat but, I like to have a second (first for me) opinion from friends here.
Thanks

[CaptPatrick]

Luis,

Yes, it's a legitimate technique.


The reason that most gelcoats remain tacky after they cure is that they usually do not contain any wax in the formula. You can also add a product to the wet gelcoat called "surfacing wax". Once wax has been added to gelcoat, the wax migrates to the surface during the cure and blocks air from contacting the gelcoat.

If unwaxed gelcoat cures in contact with air it will remain tacky almost indefinitely. That can be a good thing when you're applying more than one coat at intervals. A bad thing if you are applying a single/last coat and intend to sand it after it cures.

The other way to block air is cover the freshly applied gelcoat with PVA. This is usually done with a fine nozzel spray gun and about 90 psi. Making multiple passes of highly atomized PVA over the fresh gelcoat will create an air barrier. This does have to be applied while the gelcoat is still wet and thereby can be a tricky application without disturbing the gelcoat.

I prefer to use surfacing wax, adding about a cap full per quart of gelcoat.

PVA is water soluble and can be cleaned off with warm soapy water. The surfacing wax needs to be removed with naphtha before sanding.

[Bob H.]

Capt. Pat. Is PVA purple in color? I have seen gelcoat sprayed with a purple material then wet sanded and buffed till done. Thanks BH

[CaptPatrick]

Bob,

Not sure what the purple stuff is... All of the PVA I've ever seen is green. Of course the only brand I've used is Partall, and some one else may produce it in another color.

[Thums Up]

I have seen people use a special purple dye layer for fairing the hullsides. Maybe that is what you saw.