Most LED tubes on the market are IP20 which means they are NOT protected from liquids. A more suitable tube for installation into an outdoor signs are LED tubes with a rating of at least IP44 or higher which is a protection from water spray from any direction.
If you are considering installing lighting into areas that are are damp, dusty or dirty it pays to know the IP rating. There are some great independent information to go to such as http://www.dsmt.com/pdf/resources/iprating.pdf.
A little research will save you money in the longer term by ensuring that you have chosen the right product for your project.