Do you need to sand before painting?

[Glorz]

Hi, I am about to tackle the job of painting the bedrooms. The ceiling and walls have been painted white by someone but I'm not sure they've used a undercoat as I can see dark patched coming through. Will I need to sand it all back? Thank you

[MitchellMc]

Hi @Glorz,

If the paint is in good condition with a smooth finish and no flaking, then there is no reason to sand it back. Two new topcoats of paint will likely cover the darker patches, or to ensure full coverage, use an undercoat before the topcoats. If these dark patches are particularly noticeable, you can completely block them with a specialised undercoat like Zinsser 1L Bulls Eye 1-2-3 Undercoat Primer Sealer Stain Blocker.

Here are a couple of helpful step-by-step guides: How to prepare a wall for paint and How to paint a room.

Please let me know if you have any questions.

Mitchell

[Dave-1]

Good morning @Glorz 

A few things that come to mind when considering what needs to be done before painting.

When rubbing you hand across the surface of the rooms you want to paint how rough does the paint feel? Is there any flaky bits? 

You can scrub the walls with sugar soap depending on how marked they may be. 

Does that dark patch look like its getting darker? If not maybe it was how they painted. I would always suggest a strong undercoat as too many times I have given it a miss only to have to do three coats of the topcoat...

Dave