Preparation is crucial to ensure you get a great painting result.
1. Clean your walls with sugar soap
Making sure your walls are properly clean will help the paint to stick better. A clean surface also stops stains and marks showing through the new paint. For previously painted walls, give them a wash down with sugar soap. Mix up the sugar soap in a bucket of water according to the instructions on the bottle. Put on your gloves and thoroughly wash the walls using the sugar soap mixture. Once you've cleaned the wall, dry the sugar soap off using a rag. For new walls, you can skip this step.
2. Scrape cracked paint off the walls
After your walls have been cleaned and are dry, put on your safety glasses and dust mask and lay down a drop sheet near the wall. Use a paint scraper to remove any flaky and cracked paint from the wall. Use the ladder to reach spots higher up on the wall.
3. Apply gap filler to any cracks
Mix the powdered gap filler in a bucket with some water until it's the consistency of toothpaste. Using your paint scraper, apply it to all of the cracks and chips on the wall, making sure to overfill them.
Use a multi-purpose gap sealer and a caulking gun to fill the gaps in the corners of the walls. After applying the gap sealer, use a damp cloth to wipe off any excess. This will also make it smooth and push the filler it into the corners. You may need a second layer of gap filler to finish off.
4. Sand off the filler
When your gap filler is dry, use sandpaper on a hand sander to get a smooth finish. Remember to wear a dust mask when you're sanding. Wipe away any dust and your walls are ready to undercoat or paint. - MitchellMc
A good quality primer and undercoat should help you to get the best possible result. Just remember not to mix and match paints. So you'll want the same brand of product for the topcoat. - MartyH
If you’re having trouble with a wall because the surface is difficult based on past painting, I would seal first with a good quality 3-in-1 primer, sealer, undercoat. Then top with some Gyprock topping compound. Then sand lightly with some fine sandpaper. Then another coat of psu. Then apply topcoats. Make sure you leave plenty of drying time between coats. - Rick64
We were looking at painting over a badly painted roof but we ended up scraping the whole roof back and starting fresh and I’m so glad we did. We finished it on a long weekend and it looks great. - Moosdad