spray pva on spray gel

[nevilleh]

Just wondering if any one has any experience of spray pva after spray gel (when applying on top of the surface , not mould layup)?

I am finding that when puting wax solution in i am not getting great barcols, and if i dont put wax in then we get the surface tack which is a night mare to sand out over such a large area.

any ideas?


Neville

[Herman]

Consult your supplier. You definately should get decent barcol readings, even with wax. How thin do you spray your gelcoat? And how much wax do you add?

[nevilleh]

Herman,

Our supplier wont tell us what to expect as we are not using the contact moulding process the gelcoat was designed for (probably their way out of any liability!)

I have used wax at 1% as they said I should keep it low or no wax at all.

When I have used PVA I m getting low (10 - 15) barcols after 3 days.

In saying that I maybe applied the PVA a bit too soon . ie before the gel was going off.

Probably going to do a test piece again with wax.

Thickness is .6 - .8 mm which will allow for it to be cut back a bit.

I havent thined the gel at all because it was spray gel.

Neville

[ondarvr]

Here's the issue with post applied gel coat, 99.9% of all research and testing is done for it to be used as an in mold coating, so in a post coat application like this you won't get many guarantees. The fact that gel coat can be used like a paint is sort of an added benefit that works most of the time.

Many additives in gel coat can inhibit the backside cure, some fisheye eliminators and air release agents can do it, so can some pigments.

These things have much less of a negative affect when it's used a in a mold.

The other problem is that when styrene levels are lowered the gel coat is even more affected by air inhibition, and in your part of the world emissions are regulated to very low levels, which means styrene levels may be low. Plus 1% wax is on the low side, typically it's at least twice that. I must say though, I don't know the wax to styrene ratio of what you're buying, so it could be correct, more styrene will help the backside cure though.

Catalyze the gel coat at the upper end of the range, use a Patchaid type product if possible. If you plan to use PVA don’t wait very long before you apply it, you want the gel coat to crosslink with itself right from the start, not trying to crosslink with the air first and then be sealed off by the PVA later. Applying the PVA right after you spray the gel coat will also trap the styrene in the gel coat and not allow it to evaporate, this helps the cure. You also need to apply the gel coat thick enough, if you plan on sanding, 30 plus mils may be needed and this will help the cure.

One other thing, acetone can inhibit the cure, you may not be adding acetone, but some companies in Europe do, I can’t say this is a big issue, but everything adds up and has an effect.

[tunnels]

Sounds like you have too much wax solution in the gel coat if it's not going hard properly !
The wax is just there to float on the gelcoats surface and seal the air away .
All things in modiration !!!

[nevilleh]

Hi Tunnels, That is why we kept it down to 1%. However Ondvr is correct, gelcoat has not been designed to be post applied.

I am finding after alot of experimenting and sand backs that heat may be the key. We are getting better Barcol results keeping the surface at 22 degrees for 48 hours after spraying.

I am going to do the whole lot again with infrared heaters at 30 - 35 degrees .

I am not sure if using PVA instead of wax makes any difference, the thickness is crucial and also the hardness of whats underneath aswell.

Hopefully the next time spraying will be the last time and we will get adequate barcols.

My next worry is adhesion but I dont suppose we will know about that for a few years......

Thanks for your input.
Neville