Manifoldly exhausting

I worked out that,  with postage,  it would cost over £40 to send the air inlet manifold off to GBS to be filed flat, which seemed a bit much for something I should be able to do myself, so of I went.

I started off with the best intentions - I would start with my big file,  get down to a couple of mm and then use wet and dry or emery paper or something,  to get a nice smooth finish. I made a mask out of cardboard to prevent the file scraping bits it shouldn't :

... then paper as I got lower:

Got down to a few mm and started with emery paper and then it went a bit pear-shaped. After 10 minutes of paper,  I didn't seem to have taken off anything - 5 minutes more and my patience had run out. Out came the big file again and,  very carefully,  the last few mm were done:

Despite my resorting to brute force,  this still took me about 3 hours! And I still haven't done the final fine sanding down. 

Sump tomorrow,  hopefully...