Rich colours without soggy paper with flipchart markers
Anyone who has ever written with a normal marker on normal paper has probably experienced a nasty surprise: the ink of even slightly thicker markers bleeds through the sheet and leaves ugly ink blots on the sheet below (or on the writing pad).
This would be fatal for flipcharts, because you would no longer be able to use the sheet underneath the current one - after all, you can't insert a separator sheet in a flipchart that hangs vertically just to prevent writing through it.
Nevertheless, the colours on the paper should be rich and brilliant; and this is only possible if a certain amount of ink can soak into the paper and colour it. The problem with blotches starts when too much ink dries too slowly and the sheet then lets it pass through.
The paper of flipcharts (i.e. its quality!) is one approach to avoid the problem; but more important is the pen used: it must deliver enough ink for a brilliant typeface - but the ink must dry super fast without soaking the paper.
And that's why you use special flipchart markers, not ordinary felt-tip pens!