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