Sunscreen and Shabbat

This is Chaya Kanarfogel (first time question-poster, long time why-aanot fan): I have a fairly random halachic question about Shabbos/Yom Tov. So we were just learning in health class all the dangers of being exposed to UV rays. I thought back to this past Shavuot when it was definitely over 90 degrees and I got a sunburn after walking outside for just 10 minutes. Putting on suntan lotion on the chag had never crossed my mind. Would it ever be OK to put apply suntan lotion on Shabbos, or should I just stick to wearing a hat if it's sunny outside?
Hi Chaya! Great question! As much as the MachHach-Machal counselor side of me leans towards pushing hats/sunscreen/giant bottles of water onto people no matter what the situation, you are right that certain types of sunscreen might pose a halakhic problem on Shabbat. If the sunscreen is a thick cream or ointment, then it would be problematic since putting it on would fall under the מלאכה of ממרח - smoothing, which is a תולדה of the מלאכה of מוחק - erasing (רמב"ם הלכות שבת יא:ו and Gemara Shabbat 75b). But, if it is a liquid spray (they do make sunscreen like this), then it would be fine, as you're not spreading anything into/onto your skin. Putting on this type of sunscreen would also not be a problem of רפואה or using medicine on Shabbat, since it is preventative and acting as protection and not for healing, which is fine (שו"ע א"ח הלכות שבת ס' שכח:כג). If someone has a unique situation and even liquid sunscreen won't stop them from burning, then they should probably consult their friendly neighborhood Posek.