No worries - this topic has been discussed before so the Search function above might show you the previous posts. The main point is that Magic Supply companies and shops will sell you a spray to do the job but if you can find a shop selling model cars, boats etc, just ask them for Spray Varnish - Humbrol is the usual brand. It's the same product and is not so expensive.
Don't worry about your English, sorry for my complete lack of German!