If you have a grainy screen, you bought the wrong screen. A good screen should be a smooth white surface without any added surface coatings which may impact the image. This is why ambient light rejecting screens tend to be so miserable for image quality. But, a good flat white screen with a good border can be had for not a lot of cash with screens like the Elite SabeFrame or the SilverTicket screens (check Amazon).
IME, these look very good with a good surface that has little texture, no hotspotting, and no sparkles.
Of course, you can paint your wall if that's your preference. Painting a wall works well, but you don't have that nice border that frames can give you (unless you build something). As well, most walls aren't flat the way screens are. There are some good DiY screen building pieces of information online if you search for them.
DiY is great, and depending on your technical prowess and free time, you can do amazing things. Be aware that it can take hours, or days, to build a screen, so the time you spend building a screen yourself, or painting a screen to perfection yourself, may have been better spent just getting a fixed frame screen online.