P.C_simpson said..
Yes, lets say you cut a bit of glass and wet out the whole thing then twice more to get 3 layers, this will leave a hard edge of laminated glass, when you are feathering the edge of the glass you will be basically sanding the new 3 layers to blend it to the original glass sanding back and forth over the edge of the new glass, before the edge is feathered out and seamless into the old layers you would have most likely sanded threw the skin of the board and this will have to be repair too..
When the glass is not wet to the edge you don't get that hard edge. so sanding is much easier and feathers seamless without sanding into old skin to aggressively.
I just try and stagger the layers of glass so the edges never align at the same place.
I also like using an over sized piece of peel ply on top of the glass which I tape down and tension to keep the pressure on the repair.
If you have gone through into the foam and removed any bits of broken foam mix up a little q cell and fill any holes then glass over while its all still wet.
If done well saves quite a bit of sanding and makes for a stronger job, plus is faster.