Folding and gluing the ridge linesOn the side with the ruled lines, run a line of tacky glue down the foldline.
Fold it on the line, to spread the glue.
Now bring one of the outside foldlines (the ones marked at 4mm) to meet the other. When it is in place - this may take a little patience - weight it down along the line until the glue has 'taken'. This could take anywhere from 5 minutes to 30 minutes depending on temperature and dampness of the climate.
You will have a little pleat under the weights - it doesn't matter which way it sits.
This is what the sheet looks like on the painted side when dry.
Now you can fold the ridge pleats the other way, then backwards and forwards, running your finger along them, until they sit nicely upright with a flat background.