An oft-asked question. Many people or outdoor centres have stickers on climbing helmets - but should they? What's the answer?
In general, no you should not put stickers on helmets - but there are caveats to this. Why?
Most people will tell you that the glue on stickers is bad for the helmet. In fact, this is possible but not really the major factor. Most glues are OK but of course who knows which ones? Some glues are harmful and it's hard to know which glue is on your sticker - it's hardly foremost in your mind! So, glue is a bit of a problem and unless it's the manufacturer's own sticker (see below) then assume your glue is not suitable.
Stickers make it hard to inspect the helmet as they conceal areas and these could have problems like small cracks.
One major factor, almost always overlooked, is that of vibration. Helmets are designed and tested to withstand impacts of objects from above - in principle, falling rocks. The impact is dissipated when the shock waves travel across the helmet - but if these waves are disrupted, the helmet may not be able to withstand the impact. Cracks occur in tests around the edges of stickers.
So what's the solution?
Can I write on the helmet with a marker pen? - No, for the same reason as glues.
Some helmet manufacturers have now taken this into account and make helmets with sticker areas, so try one of those if you really want to mark your helmets. Petzl, for example, supply stickers so that you can mark and identify helmets.
If you do need to mark a helmet, for example to identify its serial number in a centre, then choose a non-critical area and write on it with an approved marker pen - or attach a tag of some type like a coloured cable tie with label - this won't affect anything.