I hand painted my yellow 90 in an oil based matt Nato green paint about 4 weeks ago and it looks brilliant. It took me about 4.5 hours to sand the existing paint so that any bubbling / high spots were flattened off, but you shouldn't need to sand it any further than removing the shine / dulling down your current paint. If you sand it right down to bare aluminium, you'll need some aluminium primer to make sure the new paint sticks. I ordered a 2.5 litre tin from this company here, and also bear in mind that Olive Drab is different from Nato Green - Nato Green being a slightly darker colour:
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and it took me about 4.5 hours to cover the whole vehicle (including the roof) using a gloss radiator roller (about 4 inch) and a decent quality paint brush (about £4) for the tricky bits. No need to thin it down, just slop it into a rolling tray and away you go - it will push quite a long way and cover quite a large area. It's a really good finnish, it dried in no time and it only needed one coat to cover the yellow. I still have about 1/3 of the tin left too, it really does go a hell of a lot further than you think. Also, just in case you're wondering - it resists oil spills very well for a matt paint, water pools brilliantly and oil just wipes straight off! I don't know how but it's brilliant stuff.
-Pos