Do I need a primer under this paint?

No on most surfaces. It self-primes on bare drywall, cured plaster and sound existing paint. Prime glossy surfaces, bare wood, metal or stained areas first.

How long before I can paint the second coat?

Wait 4 hours between coats. Touch-dry takes 1 to 2 hours, but the recoat window is what matters for adhesion.

Is it good for a bathroom with a shower?

Yes. The mildewcide and acrylic film handle humidity and occasional splashes. Run the exhaust fan and wipe standing water to get the most from it.

What does Base 1 mean for color?

Base 1 is the lightest tint base. It takes pastel and light-mid tones well but cannot reach deep or saturated colors; those need a darker base.

Can I use it on kitchen cabinets?

It can go on previously painted cabinets after cleaning and light sanding, but a dedicated cabinet or trim enamel cures harder and resists sticking better.

Soap and water while the paint is wet. Tools clean easily; dried film needs scraping or a solvent.

Where is the W3221X-004 - Kitchen & Bath Satin / Base 1 - Quart cheapest?

At Cowls Building Supply, for $27.99, as of our latest check.

Where can I buy the W3221X-004 - Kitchen & Bath Satin / Base 1 - Quart?

7 stores we checked sell it, including Newton’s Decorating Center, Town Line Wallpaper & Paint, Cowls Building Supply, Benjamin Moore Paint Outlets, Custom Colors Paint & Decorating.