Builders' Forum Archives
Re: Weave fill i sanding
Posted by Dave Houser on Sep 20, 2004
There are three kinds of dots encountered when sanding fill coats.
First is the shiny dimple, the lows in the weave than do not get touched by the sandpaper. The solution is to sand more until they are all gone. If you hit the glass before they disappear, more fill coats are required. Fill coats add to the highs and lows of the cloth so the texture of the cloth does telegraph into the fill coats and the initial sanding. Watch for the white checkerboard to know when you touch the cloth more to follow.
Second is the pock. This is a bubble in the epoxy that gets the top half sanded away leaving a sharp walled hole that catches and holds the white epoxy sanding dust. The dust seems to just stick in the pocks and will not wash out of most of them. Toothpicks tooth brushes, compressed air, etc. is necessary to remove the dust. If you are touching glass near the pocks more fill coats are required to fill the pocks. Varnish will not do it.
Third is sanding into the weave. This appears as little white checkerboards. They go away when wet so I always dry sand epoxy to know when I am into the cloth. The checkerboard does disappear with additional fill coats and hides well under varnish. I always suggest a new builder feather the raw edge of an overlapping seam in the cloth before sanding the single layer cloth areas. When feathering a cloth edge you want to feather the raw edge of the cloth away, which gives a first time builder a visual to know when he/she is sanding into glass cloth.
The ideal amount of sanding is to leave a micron of epoxy covering the glass. You just don’t know when you are there. The reality of it is the cloth never lies perfectly flat. It always floats in spots or is high at bumps or wrinkles. So I ROS sand with 80 grit until I just touch the cloth at just a few highs scattered around the entire boat. Then add a couple fill coats and wait the long wait for it to cure then sand with the fairing board and then 100 and 220 grit. You can skip the fairing board if you don’t mind the waves in the final finish created by ROS.
Wet sand the varnish when you get there.
In Response to: Weave fill by Chris F on Sep 20, 2004