Although many have tried, you cannot fill cracks and holes with paint. After the surface is clean, it's time to smooth it out by patching up. Taking the time to properly prepare surfaces before applying paint will reward you with a smoother and more even finish, while failing to prepare properly will give you a disappointing result that you'll regret.
A crack doesn't necessarily mean the wall is about to fall down, and most plasterboard walls develop the odd crack or fractured joint due to movement. To locate the smaller cracks, run a torch over the surface to find those that aren't obvious in natural light and circle any problem areas with a pencil. The next step is to follow the instructions below. Most holes and cracks can be filled using a rapid filler. Choose one that is easy to apply and dries fast. After the patching has dried just sand back as necessary.
Simply squeeze a narrow line of acrylic sealant directly over the crack then mould into the gap with a moist finger. The sealant remains flexible while keeping the crack hidden.
Use plasterboard patching cement. First dampen the crack with a brush dipped in water to allow the compound to dry gradually. Next undercut the edges of the crack so the compound is keyed in, and then follow the package instructions on when to paint.
Using a utility knife carve a 10 x 10mm deep V in the centre of the crack and fill with setting compound. The next step is to embed paper tape into taping compound over the patch, and finally overlay with two wider layers of taping compound.
For gaps where two materials join such as around door frames, architraves, cornices and skirting boards you will need a flexible sealant. Choose one that is easy to apply and is paintable, durable and cleans up in water.
Small holes and nail pops can be caused by fasteners driven too far through plasterboard or a gap between the board and wall stud, and movement of the wall can cause the filler over the screw to pop out. To remedy this problem add a screw near the first to hold the plasterboard. Next tap the hole with the handle of a broadknife or similar to make it bigger, and then cover both screw and nail heads with lightweight wall filler.