  Blowouts can occur when a dense shell confining hot, high pressure, gas
ruptures. The venting gas inflates a blister on the surface of the shell. Here
we examine the growth of such blisters on the surfaces of wind-blown-bubbles
(WBBs) and supernova remnants (SNRs) due to shell rupture caused by the
Vishniac instability. On WBBs the maximum relative size of the blister
(R_bstall/R) is found to grow linearly with time, but in many cases the blister
radius will not exceed 20 per cent of the bubble radius. Thus blowouts
initiated by the Vishniac instability are unlikely to have a major effect on
the global dynamics and properties of the bubble. The relative size of blisters
on SNRs is even smaller than on WBBs, with blisters only growing to a radius
comparable to the thickness of the cold shell of SNRs. The small size of the
SNR blowouts is, however, in good agreement with observations of blisters in
the Vela SNR. The difference in relative size between WBB and SNR blisters is
due to the much higher speed at which gas vents out of WBBs, which translates
into a greater energy flux through a rupture of a given size from interior gas
of a given pressure. Larger blisters are possible if shell ruptures are bigger
than expected.
  We expect the observed velocity structure of SNR shells to be affected by the
presence of blisters until the shell is no longer susceptible to ruptures,
since the initial expansion of blisters is faster than the ongoing expansion of
the shell.
