If we were to rank the five most important rules for skin care, they would be:

1. Buy sun protection.

2. Don’t forget to use sun protection.

3. Apply more sun protection than you think you need.

4. Don’t forget to reappply sun protection.

5. Do not underestimate the power of sun protection.

You’re not just using it to protect your skin from cellular damage, better known as the signs of aging, you’re doing it to protect your skin from skin cancer.

The fact that it keeps you looking pretty is a bonus.

Although sunscreens are widely promoted and relatively cheap, skin cancer is on the rise.

So when it comes to which sunscreen is right for you, remember to look for something that provides “broad spectrum” protection, meaning it blocks both UVB and UVA.

The SPF number gets the most hype, but it’s not the most important.

SPF only relates to sunburn.

SPF works like this: If you start to turn red in the sun after 12 minutes of unprotected exposure, you would be protected 15 times longer with an adequate supply of SPF 15 (that would be about three hours). That sounds like a lot of protection, right?

However, the key word is “adequate.”

Do more than dab it on

Experts agree that it takes one ounce or a shot glass of sunscreen over your entire body to protect you. A cotton T-shirt is said to have an SPF of only about 5. Clothing is not foolproof sun protection.

And there should be a generous quarter-coin sized amount of sun protection for your face.

Most of us don’t use that much because that’s a lot of goop on your face, but when products are tested they are not dabbed on in small quantities, according to Dr. Greg Barnum, a facial plastic surgeon with Washington University.

So when we blot on a little SPF 15, the benefits may be significantly lower or nonexistent if you wait too long to reapply or forget to coat your forehead or wash it away with perspiration, Barnum said.

And then, just to give us something else to consider while sunscreen shopping, Barnum said that the UVA rays that are not factored into SPF numbers can often be more harmful.

UVA rays penetrate deeper into the skin and are 20 times more abundant than UVB.

UVA rays are linked more closely to melanoma cancer and are present all day, every day of the year no matter what the weather. UVA rays can pass through glass in offices and cars. UVA rays are vicious.

But the active ingredients in a great many sunscreens don’t actually target UVA rays.

Zinc oxide, avobenzone and titanium dioxide are the three ingredients known to offer the broadest range of protection for UVA and UVB, listed in order of strength. And there are different variations, such as clear zinc oxide, that capitalize on the protective qualities without the chalky side effects.

“There are two types of aging: intrinsic (aging that just happens over time) and photo aging (ultraviolet-induced exposure),” says Dr. Lynn Cornelius, a dermatologist specializing in skin cancers at Washington University. She said that if you never saw a ray of sunlight your skin would age, but with photo aging, many things are accelerated, particularly ultraviolet damage to the top layer that can lead to skin cancer or the damages to layers below the skin that can break down collagen.

“The answer is sunscreen, sunscreen, sunscreen: 1, 2 and 3,” Cornelius said.

She said that if your daily routine is getting up in the morning, dashing to your car, heading to an indoor office and other typical activity, moisturizers with SPF 15 are fine. If you’re outside doing activity, use an SPF 30 and reapply every couple of hours, especially if at a pool with more exposed skin.