Painting is a messy job. Even the simplest paint jobs are bound to require a fair bit of masking off to help control the mess. In general, the more time you spend prepping and masking off the painting area, the cleaner the job will be. More thorough prep work will also speed up the actual painting because you won't need to be wasting time cleaning up spills in the middle of painting. This article explains some helpful paint prep tips for masking off vertical interior walls.
Covering the Floor
The first thing you need to do is cover the floor. Don't think you can get away with just covering the section of the floor directly underneath the spot you are painting at the time. You will have more success if you cover the whole floor at once. Also, remove all of the furniture so you have at least 4' of clear space around the wall. This creates a little walkway that makes the painting much quicker. You can use large sheets of painter's plastic to cover large floors in no time. However, plastic can be slippery when it covers a hardwood or tile floor.
Using construction grade painter's paper is a good alternative that will give a little more grip under foot. However, large paint spills on the paper can soak through and stain the floor underneath. It is important to tape your paper and plastic down to the floor very thoroughly. You want to create a stable surface that won't shift while you are painting. Try to tuck the edge of the paper underneath the molding. This is easy on carpet floors, but the edge of the paper will probably pull out when you walk on it, so use tape too.
Covering the Baseboard
Covering the baseboard is also a significant task. You will need to precisely place tape along the top edge of the baseboard. However, paint from the walls can still drip and hit the baseboard. Using a hand masker is a great way around this problem. The masker attaches a roll of thinner paper to the painter's tape as you roll it onto the baseboard. This covers the entire baseboard and even the edges of the floor. It becomes very useful on carpet floors where the masking paper is always shifting away from the baseboard when you walk on it.
Once your floor and baseboards are covered, you can begin to paint your walls.
If these tasks sound too daunting to do on your own, consider painting service by University Painters or another similar company.