I have extended recently and there are quite a few hairline cracks on the hardwall internally. My engineer tells me this is normal as the extension is tied to the original build. Is there any specific product that I can use to fill these cracks?
Check out the polyfilla website for videoguides and info.
I would also recommend "Toupret" as a very good filler.
A good painter would also fill the cracks before painting the wall.