Ink degrades in UV light. Allflex pens have xylene in the ink which carries the ink into the plastic. Black is best. White ink does not soak in as well.
There are a couple of ways to get the most out of an Allflex ink pen.
1. Wipe tag with alcohol to get the oil film off the tag or some people place the tags in a dish washer with hot water (do not use the dry/heat cycle!) The key is to get the oil film off the surface. Oil acts as a barrier and will not let the ink soak in. Plastic is made from oil and oil will constantly surface on the tag. This needs to be done each time because if you let the package of tags sit for days, oil resurfaces.
2. On dry tag, mark the number at least 3 times and let dry a few seconds between each mark in order to put a higher volume of ink on the tag. Sometimes, the ink will go completely through the other side of the tag using this method. Typically, it will last up to 4 years depending on the environment. The sun will still continue to fade the ink.
A Sharpie ink pen will last about 3 to 4 months.
Applying ink with an Allflex pen only once without cleaning the tag will last about 6 months to a year or so.
Spraying or misting marked tags with fly spray will remove ink and decreases the life of the mark.