Can you repaint dirty walls to save the trouble of cleaning them first?
Question:If you can, do you have to do any cleaning. There is a dirty nasty Apartment that has gross stuff all over the walls. I'm going to paint anyway. Should I clean them at all? DO I really need to prime first?
Answers:
Yes, you need to clean. Paint will not adhere to grease.
Yes, you need to prime, for the same reason--your primer coat helps the new coat of paint stick to the old coat of paint.
Do you REALLY want to be called back in three months and be required to repaint the whole place, because you didn't do it properly to begin with?
Yes, you need to wash with a good cleaner like lysol disinfectant, especially if there is nasty stuff, it might leave residue and particles that you might pick up on the roller and mess up your painting. If it looks like mold or mildew you need to mix bleech and water half and half and put it in a spray bottle and wipe it down.
You also need to primer it, preferably with an oil base primer I use KILZ brand, this will seal out anything (like colors) that could bleed through the paint, and also any odors.
You absolutely must clean. A good detergent (TSP would be best) mixed with water, then rinse completely. A disinfectant will not do the job properly and may leave a film behind that can react with chemicals in the paint. Never anything with ammonia.
There is no need to prime a previously painted surface. The purpose of a primer is to seal a porous surface, such as bare drywall. A previously painted surface is already sealed. A primer not bond properly to a surface that is not porous enough.
Kilz and other stain blocks are an exception, they are formulated to bond to most surfaces, porous or not. It is not necessary unless there is a stain that will bleed, such as water or smoke stains. If the new paint is a strong contrast over the existing paint (blue over orange, anything over a bright red, etc) it can help as well. If there is a slight contrast, a good paint will cover without a stain block.
Even stain block will not help if you do not clean. If there is dirt or grease it may bond to the dirt, but sooner or later the dirt will pull loose and take the stain block and paint with it.
Mop it with an alkalai degreaser just as you would a floor. it's messy, but very easy and effective.
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