gelcoat and resin laminating: time between layers

[nigelmercier]

When you fill the ground out area ... the first layer of glass should be the overall size of the chamfered area, then you add progressively smaller pieces, the final one being the original side of the hole.

Doesn't this lead to air pockets around the sides? I would have thought that the first layer of glass should be the size of the hole, then getting larger to match the edge of the chamfer.

[Later] The penny has dropped, I was wrong. I was thinking of a slight chamfer to the edge, I think VicS is talking about chamfering the edge down to almost nothing. And as Pete says below, the fibres get mixed up quite a bit.

 

[prv]

Doesn't this lead to air pockets around the sides? I would have thought that the first layer of glass should be the size of the hole, then getting larger to match the edge of the chamfer.

Personally I've never seen that it matters much, since the mats revert to individual fibres once the binder dissolves and you squodge it all together into one mass with the brush and roller. But the instructions I've seen are always to start with the biggest patch, to stick to the old structure all the way across the bevels, and fill in the dimple in the middle with the smaller ones.

Pete

 

[nigelmercier]

fill in the dimple in the middle with the smaller ones.

Thanks Pete, these words made me understand. I've added to my comment to this effect.

 

[laika]

Actually the picture in my book shows what Nigel was originally suggesting (smallest first). Due to me possibly underestimating the thickness I have to fill (mr. westerly didn't do things by halves) My initial increasingly large pieces have still left a dent which I'm going to fill with a new set of decreasingly large pieces when this layer becomes tacky...

 

[laika]

What did I do wrong?

Thanks to all who replied. A mostly successful exercise except...

I must have been doing something wrong with that first wax+pva layer. Not only was it impossible to "paint" gelcoat onto that surface, I now have lines of the blue pva stuff etched into the surface of the gelcoat, presumably where it got mixed up with the gelcoat I was trying to paint on. It's pretty deep and hasn't gone away with basic sanding.

My plan is to grind out the blue crevasses and paint another layer of gelcoat mixed with wax in styrene from outside, then sand that down (which should hopefully work) but for the benefit of anyone else reading this (and me if I do something similar again)...what did I do wrong? That wax+pva layer seemed way too slippery for the first layer of gelcoat and getting mixed in was almost inevitable.

 

[srp]

The pva is probably not necessary provided you've got a good wax surface. It is quite difficult to paint on a thin even layer of pva as it naturally wants to bead up on the wax surface, and if it isn't completely dry when you put the gel coat on you'll get the furrows you mention.