I have a join in my house where they have joined a gyprock/plasterboard and cooncrete wall. i am repainting and need ideas how to fill the tiny gap that seems expand and contract in different weather. The gap moves from nearly completely closed to about 2.5mm in width.

Does anyone know any tricks of the trade to either cover it of fill it where i can paint it and the paint won't crack

Sillicone sealant is probably the most overused option, but if the crack goes from being very small to 2.5mm you might not be able to squeeze enough into the crack for it to work properly - it will open up and peel off one surface after several months.

You've got two very dissimilar materials, with different shrinkage rates - I'd suggest attaching a piece of quad mold or similar thin timber - or even L shaped PVC edging - tight into the corner, but fastening it only to the concrete wall.

That'll hide the joint, but as it isn't fastened to the gyprock it will let the timber move as it likes. Sillicone is ideal for this sort of fastening. Might not be the look you are after, but it will reliably cover the crack till the wall falls down.

If you want an exercise in masochism, and assuming the concrete wall has a smooth finish, you could try using paper gyprock tape and joint compound. You might be lucky and find it can accomodate the movement...or you might find yourself doing it again in six months.

Make one wall a feature wall with a highly contrasting dark colour and the crack will be far less noticable.

Make it a crazy colour and I guarantee no-one will notice the crack at all.

Cheers
Ben

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My glue tastes funny.

Thanks, what exactly is quad mold

You could get some matrix tape. It is just whole series of threads running at 90 degrees to itself. Paper tape doesn't really have any give in it so that will break over time giving you back your crack. Boral make some really good jointing cements which are different to the clay one that CSR does. I am pretty sure they do one that is just right for what you want expandable. Have a look at your local hardware store and see what you can find.

Studley

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Aussie Hardwood gives me goosebumps
www.bellaphoto.com.au

Quad mould - is a timber profile, often made from the exotic timber pinus cheapus and finished in splendid primer pink.

Cross section is a quarter circle, about 19mm x 19mm,
comes in all sorts of building suitable lengths.