Halfords surface cleaner. I loostened the 1 of the 6 centre bolts at a time cleaning around them with a brush. Disconnected the air intake and all the rest of the lines etc... Plenty of masking tape and news paper to avoid overspray. Didn`t require any primer so it was quik to do.
My plenum is sat in my garage at the moment so I think this may be a project for next week