I have bought a Masi fixie riser that comes with painted unmachined rims and rim brakes.This does not seem safe.Is it?
For your specific bike, the Masi Riser, there is nothing to worry about.
Those rims are from BrevM. From the review of some brevm wheels on urban velo, it says that the rims were machined before annodizing. So, the annodizing masks the brake surface, but it should work fine. Even if they were not machined ahead of time, assuming good quality control, there really shouldn't be an issue unless they were made for disc specific mountain bike or cyclocross applictions.
So long as your rims are not disc specific, but not machined, then you should be good, and it looks like that's not the case here. Many times fixed gear bikes are sold without a machined brake surface for the seamless look, especially for those who prefer to ride brakeless.
The answer ranges between "not really at first" to "not at all."
If the rim has a braking surface that was simply painted over, then your braking power is going to suck, even to the point of being dangerous, until the paint wears off. I had a pair of Stan's ZTR 355's that were like this. One race in the mud and the paint was gone and braking power was fine.
On the other hand, many rims that are painted have a profile that was never meant for rim brakes to start with. It doesn't matter how much paint you rub off of these rims as you're using them in a manner that they were never meant to be used.
To illustrate, here's a couple of rim profiles with rules of thumb:
Possibly Ok (check with manufacturer)
If the rim states that it's "disc specific" or "disc only", go with that, and don't use rim brakes on it.