Overlaying Fibre Paper with Thinfire

Some people use a 1mm or 2 mm fibre paper with Thinfire or Papyros laid on top to get a smoother surface. It also allows assurance that the air can migrate from under the glass through the fibre paper. 

However, if the Thinfire or Papyros is laid down too many times, it fills the spaces between the fibres of the paper, and resists the passage of air.  This results in the large bubbles your are trying to avoid.  Experience will show how many times you can layer Thinfire without blocking the passage of air through the fibre paper.

This effect can also happen on sand beds.  The separating powders can build up and fill all the spaces between the sand particles. This resists the movement of air through the sand.  It leads to large bubbles just as compacted Thinfire on fibre paper does.

Is this practice of repeated layering of Thinfire and Papyros sensible?  The tipping point between achieving a smooth surface and creating bubbles cannot be known with certainty.  This means there is always a risk of bubble formation.

It is possible to use a vacuum sweeper with variable suction control  to remove the Thinfire or Papyros residue.  Using the lowest power and holding the end of the hose above the paper can lift the Thinfire or Papyros without affecting the underlying fibre paper.  Then a new sheet of Thinfire can be placed over the fibre paper to maintain the smooth surface.

Repeated layering of thin fibre papers can lead to a compaction that no longer allows the passage of air from under the glass during fusing.