I have made two mirror frames painted with oil paint over papermache and sometimes the paint can crack.
If you click on the links below you can see how they turned out.

The first is the two mirrors.
http://img15.photobucket.com/albums/v45 … 0_3411.jpg

The second is the cracking effect.
http://img15.photobucket.com/albums/v45 … 0_3413.jpg

Hope this helps.
Nicky.

Nicky, I love your mirrors! What did you do to make the surface look as it does? It looks like old wood to me. Did you decoupage the fish? Are there any other papier maches you could show us? I'd really love to see them.

Thank you for your interest, as this is only a hobby I only have one more mirror.
As you can see I use a chipboard base then I add an emulsion undercoat, then oil paint (raw seinna and burnt umber) and also indian inks, then lots of sanding and more coats of paint, the paint stays in the grain of the paper causing the antique wood effect !.
The fish came about when I was looking at a recipe magazine and I thought the pictures would make make great fish, so I decoupaged them on. I am about to start on a new mirror in the next few days.

Here's two pictures of the other mirror.

http://img15.photobucket.com/albums/v45 … 0_3568.jpg

http://img15.photobucket.com/albums/v45 … 0_3569.jpg