PAINTING WALLS & CEILING

No not if you want to invite problems following advice that its ok to do so..very light sand 180 grit and wash down with sugar soap then clean water. Prep is key to any job....diy....or the difference between the pro decos and the jobbers

I'm afraid I have to disagree with these replies. If you paint matt directly on to silk you risk the paint crazing. Once that happens it is a real task to correct. There are a couple of choices. Give it all a light sand to take the sheen off and give a better key. The alternative if you don't fancy all the sanding, you can use an eggshell paint as a base which will adhere better to the silk. Then you can use matt on top of that. That obviously means buying more paint so sanding is probably more sensible - but you do need to do it. It will save you work in the long run.

yes clean down first

Sounds good! white matt is a good base coat to fill and finish

Yes

Indeed you can - wipe down the wall before applying any paint as there is always reside or unseen dirt for the best finish

Yes you can get rid of any dust or dirt