Let's start with suntan lotion. I recommend avoiding suntan lotion since the SPF is less than 15 and the purpose of the product is to promote tanning. Be careful, even if your skin tans easily and you never burn - the sun is still damaging and ageing your skin with the UVA (ageing) rays. This damage includes and is not limited to skin cancer and leathery looking skin caused by collagen damage.
Sunscreens are chemical based products that work by absorbing harmful UVA & UVB (ultraviolet) light before it penetrates the skin and converting this light energy to a less harmful form. These chemicals act as a filter to screen the sun's UV rays. Chemical sunscreens break down after prolonged exposure to the sun reducing their effectiveness which leads to a greater risk of sunburn and sun damage. Something to note is that people use the words sunscreen and sunblock interchangeably, yet each one means something completely different.
Sunblocks use minerals that reflect the sun's rays from the skin. Sunblocks physically block and protect the skin against both UVA (ageing) and UVB (burning) radiation, so the radiation does not penetrate your skin The two most common minerals in use are zinc oxide and titanium dioxide.
SPF stands for Sun Protection Factor. SPF measures how much UV radiation or solar energy is required to produce a sunburn on protected skin as compared to the amount of solar energy required to produce a sunburn on unprotected skin. The popular misconception among the public is that SPF relates to time of solar exposure. This is not the case. For example, many believe that if they normally get a sunburn in one hour, then an SPF 15 sunscreen will allow them to stay in the sun safely for 15 hours without injury. Again, this is highly misleading and untrue because SPF is not directly related to time of solar exposure, but to the amount of solar exposure - think how much stronger the sun is in the middle of the day than it is in the evening. Visit the link below for more insight.
With respect to SPF, think of it as the percentage of UVB rays blocked rather than safe exposure time. As described here SPF 15 blocks 93% UVB exposure, SPF30 blocks 97% UVB exposure, and SPF 45+ blocks 98% UVB exposure. Anything above an SPF 30 does not afford anymore UVB protection and is not worth the additional expense. For protection against UVA and UVB rays, purchase a "broad spectrum sunscreen" which contains a physical sunblock as described in paragraph four.
I personally believe physical sunblocks are superior to chemical sunscreens. My personal favorite is Skin Script's Sheer Protection SPF 30. It's a physical sunblock and is reef safe. It works well as a makeup primer too and it's always in stock at my place. For acne prone skin I recommend Face Reality's sunscreens. For more information on sun protection and acne please refer to my blog-