Hi, I'm trying to paint Dulux Trade Vinyl Matt over old white paint which is probably around 7/8 years old. I don't know what type of paint was used but it appears to have been applied with a brush in very patchy, thick layers. When I apply the emulsion I am finding the paint does not uniformly take to the wall and that there are many patches in which it will not stick. I've attached an image of one of those areas. It actually looks as though this was the case for whoever painted the wall previously as well, as even before painting I can see similar patches left in the attempted brush strokes
I should also say that I didn't know before using it that you needed to wash sugar soap off before it dries. I'm mentioning it in case that might contribute to this problem but I'm pretty sure it has more to do with the underlying paint layers (Image also attached of this).
I was hoping to know how I can create a layer or 'key' that the vinyl matt emulsion will stick to. Do I need to sand back the walls and then apply a primer before continuing with the emulsion? Thanks for any help with this, I've read through a lot of the similar posts with very helpful answers but I'm still unsure how to proceed.
Jim