Mix the filler slowly, or you’ll get air pockets. I purposefully mixed this batch quickly to illustrate that. The arrows indicate the air bubbles, which will transfer to the panel the filler is applied to. They may be slightly under the surface and undetectable until they blister through the finished paintwork at a later date. Small air bubbles, which will appear as pinholes in the sanded filler, are unavoidable and can be filled with putty prior to primer, but large ones will ruin all your good work.