Washing with sugar soap
Although modern paints cope surprisingly well over a small amount of dirt, it is far better to wash down with sugar soap first.
All paintwork should be washed with sugar soap before repainting.
The sugar soap may be in powder or concentrated liquid form. This should be dissolved in water first.
Applying sugar soap
Paint on with an old brush or apply with a sponge. Work it into the dirt using a rubbing or scrubbing action.
Allow it to act for a few minutes, before rinsing thoroughly to avoid softening of the new paint.
The sugar soap actually does two jobs. It cleans away grease, dirt, and nicotine stains, whilst also etching the paint surface to provide a mechanical key.
Use warm water to dissolve the sugar soap quickly. It’s also more comfortable to work with.
Sugar soap can be an irritant. Wear rubber gloves and avoid contact with the eyes.