Matt over silk: how much sanding to do?

[Poiup]

Hi all!
I am looking at fixing the walls that have a few nicks and re-painting all of them. All walls are painted with vinyl silk and we want to replace it with a matt finish.

I am planning to sand them, fix the odd spots with easyfill and then paint.
Now... How much sanding do I actually have to do? Is it just to scratch the surface of the existing silk paint or do I have to do some serious sanding? How do I know if I have done enough sanding? I am using 80 grit sand paper. Is it coarse enough?

 

[Chivas69]

Might be best putting some bulls eye primer/sealer on before the Matt!

 

[sparkwright]

The primer sounds a good idea - I would say a very light sanding, you don't want to 'damage' the surface.

You just need to take the sheen off, so it's flat and dull.

You shouldn't be seeing very much off the previous paint colours or bare plaster - except where you are deliberately trying to smooth out imperfections.

 

[Poiup]

The reason for sanding was to avoid using a primer... Do I have to use it anyway?

They are around 15/20£ per litre and cover around 12m2... I have the whole house to do, I'd be looking at spending hundreds in the primer alone

Is there any other alternative?

 

[Chivas69]

The sanding may well work. You could try Leyland primer/undercoat which is about £24 for 5 litres and goes a very long way. Obviously if you stick with silk you have no troubles at all.
I have painted over silk with Matt in the past and it has been fine, mainly because the customer said to just do it. It's one of those where you see how it ends up but the main problem is that the paint can craze. A gentle sanding will help though.

 

[Poiup]

Here is the before and after sanding a tiny stretch if one wall. Should this be enough/too much/too little?

 

[Poiup]

For people who might find this thread in the future. Sanding is not worth it.
If you are on a tight budget like me: just paint matt over vinyl silk. IF it will bubbles in some places, give those spots a light sanding and paint over them again with matt. Job done.