Hi Ted,

"Roughen it up with sandpaper etc."

You've planted that 'etc' where a whole lot of other stuff should be.
What do you use for roughening up? I long ago gave up with the stupid bits of paper you find in the repair kits. I use a cut-up fairly coarse (don't remember the grit No.) sanding band used on floor sanders.
And rub until every trace of moulding mark, embossed pattern or seam mark has gone.
That leaves a lot of powder and grit in the surface of the rubber.
AliBaba says to use petrol, which sounds pretty good. I never do, but give the surface a thorough flicking, not rubbing, with a clean cloth. Like you're dusting the Crown Jewels......

Either way you need to get rid of all that dust without further contamination. (But I'm sure you know all this anyway).
Then proceed as you describe.

But there's one other item that I use that's missing, and may be important when you're in a hot place.
The hotter it is, the more 'sticky' or 'tacky' will be the surface of the patch and the inside of your tyre. If the two stick together just lightly and you're not on the smoothest of roads, then the patch will be disturbed. (You probably know all this as well).
So use plenty of talcum powder, or something similar.
My advice, always have plenty in your puncture toolkit, and use it lots!
Imperial Leather works for me!

Hope that helps.
Enjoy Tanzania, for me it was a great place.

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TTR250 - London to Cape Town