Is your Halfords Valve Grinding Paste approved for use on aircraft???
Screw Grab should be your first resort before drilling/easy out/imapct breaker, especially if the panel underneath is composite, unless the recess is totally chewed up. Shouldn't use a impact breaker/rattler/T Bar snap on those bar last resort. Sorry, second after you pick out the paint/rubbish from of the screw first.. I hate the people who drill off the head and leave the stud in the nut!
