How can I prevent solder from breaking off in the through-holes?

Believe it or not, the way you hold the bottle can make a big difference on how easily the cured mask is removed. Many operators hold the bottle at a 90° angle as they drag the dispensing tip across the area they are masking. This has a tendency to force the mask through the vias and causes 3 potential problems:

1) The mask tends to stick to the inside of the thru-holes, and may push out the other side and mushroom out. This creates a plug on the other side, which increases the chance of breakage when peeling off the cured mask.
2) The material inside the thru-holes are less accessible if washing off mask in an aqueous cleaning system.
3) The bead of mask is thinner when holding the bottle at 90°.

When using a peelable mask, this bead acts as a draw-string, so a thinner strip has more of a tendency to become brittle in high soldering temperatures, and is more likely to break when removing the cured mask.