Persistant cracks in window sill joints.
 

Hi All.

Could anyone help with a very annoying problem.

I recently decorated my dining room which has a five sided bay. I ripped out
the very old window sill and made good
the plater underneath that broke away with plaster board sat on top of
plasterboard bonding (the whaite doughy stuff).

I made the window sill out of cill board (mdf pre coated) and stuck it down
to the plasterboard with a polyurothene adhesive that is used to fix/glue
buses together with. It's basically a very adhesive silicon sealant and
certainly better than no nails.

All went well except every couple of weeks I get the paint cracking at the
joints. I've used decorators filler. But they re-appear.

I'm thinking expansion and was going to drill through the would and screw
the sill in place.

The paint is crown solo - it's very thick like emulsion, and i've given it
about 4 sand and coats now...