Bubbling paint.

[woodchopper]

My garage was built about 10 yrs ago. It was rendered and painted. A few weeks ago I noticed that a patch of paint about a foot square has started to blister and bubble. when I exmained it By poking at it. it seems that the render immediately beneath the surface has gone a bit powdery. What has caused this, and how can it be remedied. Is it a simple matter of scrapping and repainting or will it need to be sealed before painting ?
I realise that if i scape it and then repaint it will have uneven edges.

[The Builder]

A lot of render has gone like that, woody............ as a result of the hard winter. It might well be frost damaged. Just hack off the loose and patch with new render.
Putting a little fine sand in the paint helps to "lose" the join.

[AlBags]

DIY Sandtex : )