Sanding and fairing

I built one plywood boat covered by epoxy fibergalss on the exterior. I want now to sand the hull and I was looking that I should sand the edges of fiberglass tape but the fiberglass layer is so thin that I wonder what will happen if I will sand the fiberglass and reach the plywood material being so thin.
Then probably I should fair the surface with fairing putty made by epoxy resin with cotton filler, how could I do this fairing the best using this type of putty ? Do I need to take care to any especial aspect?

 

There are fairing compounds available - sanding glass has to be avoided because of the many broken strands that will reduce the strength of the glass layer.
When the layer is set in epoxy you better ue also an epoxy fairing compound, though they are sometimes hard to sand.

 

Best way to get a good finish is to skim the surface of the hull with a mixture of light weight filler and epoxy resin. I used Q Cells for the filler here in Australia, but other lightweight fillers are made by many different manufactures. The best way (I found) is to create ridges of filler over the surface of the hull by using a grooved trowel. Once these have gone off you can sand them with a longboard to get these fair. Its then a simple matter of filling the faired ridges. Final surface should be resonably close to fair and only require a little work to get right. Finish with high build epoxy primer and a top coat of your choice. Try to avoid sanding the fibreglass - hard hazardous work which could weaken the structure. Checkout http://www.johnwatson.net.au for an over view

John