Removing Sugar Soap Residue

I'm repainting a wall and as per instructions I've washed down the wall with sugar soap solution. However, I'm having a very difficult job removing all traces of the soap.
I've rinsed the wall down with a damp sponge several times, but there's still a residue of soap left. It's not visible when dry but when you wipe the wall with a clean cloth, foam appears showing that there is still soap there.
Does this matter? Can I go ahead and paint or should I remove all traces of sugar soap? And if so, how?
thanks,
n
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The sugar soap can actually act as a sort of size for the wall, sealing it over to take the paint, so it shouldn't matter that you have some residue left. As long as it's smooth.
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