You can fix a crackle paint finish by touching up an area to even it out, or by adding another layer of crackle glaze to enhance and deepen the crackle look. However, if you do not wish to add more crackle, and still do not like the existing paint finish, the only thing you can do is allow it to dry then sand it all off. There is no need to remove all of the paint beneath the crackle -- only the texture of the crackle.
Inspect the walls you have crackled and painted to determine what it is you do not like about how the finish turned out. Skip to Step 3 if the crackle finish has been applied too thick and cannot be fixed by adding more. Otherwise, look for uneven areas which can be filled in to improve the look and then apply slightly more crackle paint to those specific areas to fix the unevenness.
Add another layer of crackle paint over the entire surface for a crackle glaze that did not crackle enough, or that appears too thin. This will enhance the crackle effect, making it deeper and richer and will cover the paint a little more.
Plug in an electric hand-held sander and put on a face mask. Open the windows to ventilate the room, and use a fan to suck the dust out of the room if you have one. Sand the entire surface of each wall if the crackle finish is just too thick and is not able to be fixed by adding more crackle finish. Remove the top layer of paint on each wall, removing only the texture of the crackle finish, not the paint below it. Sand each wall until there is no texture remaining and the walls are smooth to the touch. Dust off the walls using a soft, dry, disposable cloth.
Skip to Step 5 if you do not wish to try to crackle your walls again. Otherwise, apply a fresh coat of crackle glaze on each wall, using the same applicator you did before or another type which can give you a different crackle effect. Allow the crackle finish to dry before filling spots which need more or before applying a second layer.
Apply masking tape around the top and bottom edges of each wall and on the floor and ceiling to prevent paint from getting on either of them. Lay down a painter's tarpaulin to further protect your floor. Fill a paint tray with paint, then apply using a roller for the main surface area and a small paintbrush for corners and edges. Allow the paint to dry before applying the second coat. Allow the second coat to dry for two hours before removing all masking tape. Do this if you do not wish to try to crackle finish the room again and just want your walls painted and fixed.
Wear a mask while sanding, ventilate well.