They will know what fixer is, but the fixer they would be thinking of would most likely be in spray form, which would probably work but would also cost quite a bit. Fixer can be anything from turpentine/white spirit to alcohol to varnish and so on, it all depends on what you want to do and how permanent you want the effects to be. I've recently started to use Vallejo's pigment fixer and it works better than glaze medium which is what I had been using before. If you have turpentine and alcohol handy, I'd experiment with those to give you an idea of what they do and also pick up either a glaze medium or dedicated fixer product. I'd totally recommend the Vallejo brand, cleans up with water and no real odour to speak of. A nifty tip I found on their site was to use airbrush cleaner (not brush cleaner)to remove or blend, it shouldn't damage the paint if it has cured. And don't be afraid of pigments, they really are dead simple to use.

Kimmo