Sponge painting is one of the easiest faux finishes to do for beginners and ranges from ultra-simple to fairly complex. These effects can be used to decorate walls, furniture or just about any paint-able surface and they are particularly suited for hiding rough wall surfaces.
There are a couple different techniques that can be used to create these effects known as positive sponging and negative sponging. Positive sponging involves applying glaze, a little at a time, using a sponge to dab the color onto the surface.
There are degrees of positive sponging, using just one application of glaze for a transparent effect that allows much of the base color to show through, and multiple sponged patterns, one over the other, to create an opaque effect that nearly covers the underlying color.
In addition to the basic technique of applying random color with the sponge, more glaze colors can be added over the finish to create more complex and interesting effects such as adding a craft paper finish or a flowing pattern to create depth in the finish.
Negative sponging is so called because, after the glaze is applied to the surface, a wet sponge is used to removed some of the color to leave a pattern of light and dark areas on the surface. This technique doesn't allow for much room to be creative with the finish, but can be a good choice for a small powder room where you don't want to get too busy with the wall covering.
See this link for instructions on choosing colors, surface preparation, base coating and mixing glaze. It's best to use a natural sea sponge for this, they are better at creating a consistent pattern without adding too much glaze to the surface than the other choices. Synthetic sponges and sponge rollers are also available for this type of painting, but they tend to get clogged with glaze and the roller can be hard to control.
If you're doing a simple finish with just one glaze coat, a quart of glaze will probably be enough for an average 9x12 room. You will also need a cup of paint for every quart of glaze you use. If you're doing a more complex pattern, using 2 or more colors, use a gallon of glaze and divide it up for each separate color you will be using. Mix all the glaze you will need at one time to keep the colors consistent over the whole finish.
Mask the ceiling and around trim moldings like door casing and baseboards to keep the glaze off of these surfaces. Using two strips of 1½ inch painter's tape to create a wide border around the project will prevent any accidental stains that could be hard to remove later and would distract from the overall effect.
Wear latex gloves while handling the sponge, glaze will spread to your hands very quickly and can be hard to remove completely. Use a roller tray to hold the paint and blot the sponge to get it ready to use. Also have a bucket about half-full of clean water to rinse the sponge when necessary. Have some paper towels or dry rags ready and use them to remove all the excess water from the sponge to prevent runs that can occur if you add too much water to the glaze and then squeeze the sponge against the wall.
Before beginning to sponge on the walls it's best to practice your technique using a test board such a large piece of cardboard to get the pattern and depth of color you want. When you base coat the walls, paint the test board too and set it aside to dry for this purpose. Also have a piece of cardboard small enough to hold in one hand and use it to block the sponge at corners, this will keep the pattern off the adjacent wall until you're ready to do that surface.
When working at corners use a piece of card board or paint shield to keep the sponge from touching the other wall. Use a smaller piece of sponge at the corner if necessary to create a consistent sponge pattern.
Submerge the sponge in the water bucket to saturate and squeeze out the excess. Wrap the sponge in a paper towel or dry rag and squeeze to remove all the excess water. This is important to avoid adding water to the glaze that could cause it run when you get it on the walls.
Pour the first glaze color into a paint tray, squeeze the sponge to make it pucker on one side and dip it into the pool. Press and swirl the sponge around on the tray to distribute the liquid evenly and then dab at the test board to practice your technique until the desired pattern is produced.
Keep the sponge puckering on one side and beginning in one corner of the wall, at the ceiling, dab it onto the wall surface. Create a regular pattern by dabbing in parallel rows across the wall along the ceiling and then move down to start a new row.
Avoid rocking or dragging the sponge while it's in contact with the wall, touch the surface lightly and lift straight off in a light pouncing motion, leaving a subtle impression of the sponge behind. To create a random pattern vary each pounce of the sponge up and down slightly across the wall.
Avoid repeating the same impression over and over by working in all directions and turning the sponge slightly with each pounce. Refill the sponge as needed and dab it out on the paint tray or test board before going back to the wall. If the sponge becomes to full of glaze it can sometime leave a thicker pattern on the wall, it's a good idea to wash all the glaze out completely from time to time to keep the pattern consistent.
With an opaque sponge technique, a heavier touch can be used to apply a first glaze coats. Because a dense coat of glaze is the goal, the surface can be dabbed repeatedly in the same place. It should be a uniform coat with some dark and some lighter spot while still allowing the base coat to show through the glaze.
Do all the walls with this technique first and then come back after it dries and do another, accent color over it. Clean up the tray and sponge and let them dry as well.
When the first coat of glaze has dried, pour the second color into the paint tray and begin with the same technique of filling the sponge and testing your technique. Apply less glaze this time, using the same technique as with the transparent sponging described above. Make sure this coat allows the underlying colors of the base and first glaze coat to show through. Do all the walls and let the glaze dry.
If you're happy with the finish at this point your room is done, if you would like to add more depth to the finish, lightly sponge on another, accenting glaze color. Use progressively thinner patterns of glaze with each coat to let all the underlying colors show through. This will create a feeling of depth in the finish.
To do this effect, work on one wall at a time and coating it completely with glaze before doing the sponging part. Use a brush to apply the glaze at the corners and ceiling and a roller to do the wide space. Fill a roller lightly with the glaze color, avoid overfilling, too much glaze in the roller will put too much glaze on the wall and make it hard to remove enough to get a satisfactory effect. Roll back and forth repeatedly to spread the glaze out and leave a thin coat consistent with the brushed parts.
Soak the sponge in the bucket of water, wring it out and then dry it some more on paper towels of dry rags. Start at one corner and press the sponge lightly against the glaze and then pull it straight up to lift some of the color off. Turn the sponge to a clean spot and do another pounce. When it becomes covered with glaze, wash the sponge out in the water bucket again and dry it with a rag before going back to the wall.
Do one the entire wall like this and at the corners, use the shield board to keep the sponging from touching the adjacent wall. Cut in and roll the next wall with the glaze as before, spreading it thinly. Wash out the sponge, dry it and starting at the corner with the first wall, dab with the sponge to create the same pattern as the first. Use the shield board to keep the sponge off the finished wall and then move out into the wall and on to the next corner.