It took me about 2 hours to give the third and final coat of highbuild a light sand. I have just to sand the rounded chamfer turn. In order to sand the chine straight on the outboard side, I measured and marked a level straight line 10mm below the correct position of the chine and then marked another line 10mm above it. This created a railroad track with the chine meandering up and down by a few mm either way along the length. I then sanded the point of the chine down and bias up or down depending on where the chine sat in relation to the lines. This centered the chine and rounded it which further lessened the visibility of any discrepancy making the chine look straight.
I then washed the hull and marked the waterline. I used the 2000mm measurement above the top face of the strongback on the outboard side where I could take a measurement. On the inboard side it is not possible to do this, so I ran a stringline and measured from the middle of the chine turn to match the other hull to be sure but the stringline was in the correct position. Then using the waterline marks as the start point I used the spirit level, which is 60mm wide, to mark a line above the waterline (below the line on the upturned hull) to mark my mask line to where the copper epoxy will cover. Some people advocate a little more (say 100mm) some less, so I settled on 60mm because of the ease of marking the width of the level, using the level to ensure the line was level at the same time.
Once the mask line was marked it was then an easy task to run masking tape along the lines, to which I could attach the skirts to catch any drips of copper epoxy onto the sanded areas so that I don’t have to sand them off again.