I saw this and thought you guys might find this useful/interesting.  The video is about making a thermal epoxy, but also illustrates something interesting.  Using a cheap $3 mirror as a flat surface for sanding down aluminum.  I'm not sure if anyone would need this for sealing things.  But I still found it neat. 

https://www.youtube.com/watch?v=8MOTMq9g8Nk

Ian R. wrote:

That is an interesting video channel ! I've bookmarked it to watch a few more episodes later.

On the topic of what to use for a flat surface, I actually use a square of plate glass that is just under a 1/2 thick which I picked up a few years ago. Works great ! 

The channel is really good.  They go into a lot of very specific details, which can be both great and annoying.  But stuff like the turbo converted into a jet engine is fun.  The swamp cooler is pretty neat also.