Im putting a new all weatherproof single gang electrical box on my garage (40 yrs old) with sometime of cork frame backing. When i screwed it in it just kept on turning regardless if i had increase the screw size. It isn't catching (like putting it into stucco). There was an old one on there that had a 12 and 14 gauge illegal connection and the box was also loose. What would you recommend a particular anchor for this or just drill it through 1 1/2" and put a machine screw with a Nut on the inside of the garage ( not sure if that is code). Also because the electrical box wont lay flat due to the two brackets that protrude out (thus making it impossible to be flushed), should i add two more brackets (four in all) so that it would be somewhat flush against the siding? I don't want to put a wall mount behind it. Appreciate your help. By the way inside the garage it is like a silver paper backing. the material looks like this - similar to a plaster with a white substance and silver backing
You need longer screws that will bite into the base siding or find a stud. Aluminum sideing is very thin and will not hold well even with larger screws. Since this is a garrage and you mention silver paper backing if that is the only thing behind the sideing you may need to add a nailer between 2 studs to support the box.