Interior painting tip 1.
To tell whether your current interior wall or trim is water or oil-based, douse a clean white cloth with rubbing alcohol and rub it on the wall . If the paint softens and begins to transfer onto your cloth, it is water based. If the alcohol does not remove any color, it is oil-based.
By the way do this in an inconspicuous spot just in case you change your mind about painting your interior.
Interior painting tip 2.
Measure the square footage of the room you are painting before you head to the store. I recommend one gallon of paint for every 400 square feet. Covering textured, rough or non primed surfaces will require more paint...so be sure to mention this fact to the paint store....
To get the square footage of the area to be painted measure the length x the height for each surface you are painting and if you're doing multiple interior coats like i always recommend then you will have to double that number.
Interior painting tip 3.
If you have an oil based finish on the wall or trim now you will need to sand down the surface to roughen it up..wear a dust mask and tack cloth to remove dust...if your going back over with oil based paint go right ahead otherwise you will need to use a latex primer to cover the entire surface before you can paint with latex top coat...if you don't you are not going to get a good bond and peeling will happen.
Adversely if your applying an oil based paint over a latex wall or trim then you wont have to sand but you will need to apply an oiled based primer first.
Interior painting tip 4.
Please don't cover the floors with plastic as it can be very slippery and a broken bone stops the interior painting project pronto.. Use drop cloths or old sheets to protect them from splatters and drips.
I only ever use plastic around the edges if i am removing wallpaper but even then I put a canvas tarp over the top for safety.
Interior painting tip 5.
The color mixers at the paint stores like Sherwin Williams, Pittsburgh paints, Menards or Lowes can create slight variations from can to can. The last thing you want is to have two shades of a color on the same interior wall. This can be remedied by mixing all of the paint cans together using a larger 5 gallon bucket before you start painting to ensure you will have consistent color throughout the room.