Sanding down a machined wheel

Well it won't be stronger... it depends on how much material you have to remove to get the finish you want. If it's just a quick wipe over with some 600g then they'll be fine, but if you go at them for hours then the rim walls will be thinner and weaker (and prone to splitting under pressure from the inner tube).

I would just paint the rims, or get some track-specific rims that don't have a braking surface. A sanded rim won't really look any different to a machined rim unless you're inspecting it very closely.

do you mean as strong? cos obvs i know the rim wont be stronger, i take the braking surface off so it looks unmachined, so how every much there is, and then respray