Can't get walls smooth

[Londonideas]

I'm decorating for the first time and I can't get the walls smooth. I bought some sandpaper that didn't do much so I went for a much grittier one which just scratches the walls. I painted and the wall looks bumpy and uneven. What can I do?

When you say you used sandpaper, did you use it by hand? A power sander is much better and you can use a finer grit paper on it.
But it also depends what your walls are like, some won't be perfectly smooth and you may consider lining paper.

There is thick paint you can use as an undercoat that works quite well

What's it called please?

What was the previous wall coating like?
Any what have you done to the walls so far?
e.g. was it wallpaper or badly painted walls?

The polyfiller base coats might be worth looking at. Or just keep going with the (fine) sandpaper - it can be easier to see the imperfections once the wall has been painted!

I think there's quite a fee but someone has mentioned pollyfilla base coats. But I used a thick base coat that works really well

The common materials used to plaster walls don't sand well. Have you used filler? Good filler is very easy to sand smooth and flat (with 120 grit).

Can you post a photo?

It may be you are on a hiding to nothing.
Lining paper (different grades available) can cover all sorts of problems but sometimes you have to admit defeat and have the wall skimmed.

As per above comment... Is it a plastered wall? Have you removed paper? Sometimes you need to have the walls skimmed. This is a thin skim of plaster. You'll probably need to get a plasterer to do it. It takes a while to dry but the walls will then be completely smooth. Then, your first coat of paint needs to be a mist coat (mixed with water - see tin for ratio). Then one or two top coats. Post a photo and we'll have a look.

We have been renovating our house and had the same issues, we did not want to replaster everywhere. After a lot of research I found out about WallRock Fibreliner wallpaper.

https://www.screwfix.com/p/erfurt-white-wallrock-fibreliner-wallpaper-1000mm-x-20m/1506p

It's incredibly thick stuff, covers loads of imperfections and you can use fine filler on joins. When painted you cannot even tell the walls have been lined.

I've used that and then used some very plain textured wallpaper (a bit like beadboard) and then painted over. If all else fails a carefully positioned picture or plant works well I find!