That is strictly for fine dust and particulates, not for fumes, as previously mentioned. You need a quality respirator with the ability to filter organic vapors, the type that are of course emitted by such things as fiberglass resin and bondo. In my opinion, its definitely not something you should take lightly. Spend the extra money to make sure that you are well protected.
You've got to read those safety instructions very carefully. It says "fibreglass", not "resin".
You can use that thing wile cutting your fibreglass cloth and it will protect you from inhaling any of those tiny glass particles that may start flying around, but it will not protect you from any kind of vapour.