Sunblock Reapplication

So my sunscreen is SPF 30 and says it lasts about 40 minutes before reapplication. I'm assuming this is in the sun. If I spend 40 minutes in the sun, I should reapply.

But what if I'm going in and out? If I'm in the sun for ten minutes (walking from parking lot to class), is the degrading process beginning? Does that mean I have 30 more minutes of sun exposure before I have to reapply, or do I reapply sunscreen in thirty minutes since my exposure.

Am I making any sense?

Also, for avid users of sunblock, do you wash off the old sunblock before you apply new sunblock, or do you just layer on some more?

Switching Antibiotics

I went to my first dermatologist appointment yesterday. It was something of a disappointment. She told me to switch from erythromycin 250 mg 2x daily to soldyn 90 mg 1x daily. I've been looking into the side effects of minocycline and it appears that it interferes with the effectiveness of birth control.

First of all, my acne is hormonally triggered, which I tried to explain to my derm but was shrugged off. The reason why my acne cleared up in the first place was because I went on the pill.

I'm also tentative about being on antibiotics for so long. I've been on erythromycin since Jan 9 and it's been effective. Hardly any new spots. I don't want to switch antibiotics and get another initial break out. Or does it even work that way?

Also, I might go into the jungle (Ecuador) this summer for some school work and I'll be taking doxycycline for the duration. Three different antibiotics in half a year? It just sounds troublesome to me. When should I stop taking antibiotics?

Any advice, suggestions, or comments would be appreciated. Thanks