Aluminium box section

People seem to be getting good results with just the manufacture recommended mix of 209. Then again it is a bit of a ghetto solution for people needing a flat edge when they don't already have one and are unwilling to spend hundreds/thousands to get a straight edge.

The only other way I know of getting a cheap flat edge is the method used for hand grinding optical flats, (people hand grinding their own telescope mirrors need tolerances that make machinists look like cowboys) the downside of that method is it means weeks and weeks of boring repetitive manual labour. By the time you factor in the cost of all different grades of grit and building a jig it would work out more expensive then letting gravity do the work.

People seem to be getting good results with just the manufacture recommended mix of 209. Then again it is a bit of a ghetto solution for people needing a flat edge when they don't already have one and are unwilling to spend hundreds/thousands to get a straight edge.

Not a case of being unwilling to spend the money required to get a level surface, I simply don't have hundreds/thousands of pounds so the options are more limited. Now I have something else to explore having read your last post DC thanks !!
Don't know why I didn't think of it myself, one of my customers has a business grinding optical lenses amongst other things, They bought some machinery from a liquidation auction a year or so ago that I think is just in storage in case their own machines have problems. But if any of them are hooked up & I ask very nicely there is a chance that they might let me use one of them, sure there must be something I could offer to do for them in return.