Vortex Fan, Stubborn screws

Hi, I'm trying to install a 6" in-line Vortex fan in my grow tent. I want to attach the brackets to the sides so I can secure it in the tent more easily. The trouble is I have to remove the screws from the sides of the fan first and they're too tight. I've tried applying WD-40 and letting it sit for a few hours but only managed to get one of the four screws to loosen up a bit. I applied another healthy helping of WD-40 to the screws and I was going to let it sit overnight and then try it again. I was going to use a socket wrench this time to get more torque. I'm concerned about damaging the screws and possibly voiding my warranty though. Does anyone have any further suggestions. I'm anxious to get this thing set up so I can install my 600W HPS.

Wow, that worked great. Thanks for your help. Can a PM you with any further questions?

Painted over?

Either way try this. Set on a solid surface. Put screwdriver in screw. and give a good wack, but not a pounding or tap to the screwdriver with a hamer.
Sometimes the rapid jar breaks things free.

Okay thanks, they weren't painted over but between your suggestion, the WD-40 and the socket wrench I managed to get the screws out. The good news is that they were so tight that the brackets attached to them won't be going anywhere. It should be pretty easy to secure in the tent.