just use the normal hobby primer...

nope.. i've try before... normal cheap ass hardware primer wont stick to it...

for brake disc, the best would be get a photo etch brake disc. Of course that ll be for those who can afford it.

to cut a perfect round thin pla plate, i m not a fan of hole punching, which i feel the edge is too much trouble to fix, and sizes r limited.
get a compass, try to get those better ones where u can lock it. use the sharp end to scribe line on the pla plate, slowly do it, like u r scribbing panel lines. but remember, u cannot scrib until the end where the pal plate will seperate. cause first, it will take forever, second, the line will be distorted by the thickness of the pla plate.

After u scrib quite a deep line, use cutter to cut a straight line, example:


Then u can actually peel off the excess pla plate, last step will be sanding off thosee extra fine pla plates. with the line scribbed earlier, use that as a guide so your circle wont be off.

for disc brake....just tot of something u can use. go buy button!!! variety of sizes n types. get round ones...with minimal groove. kinda like those u have on school uniform. seal n touch up from there. i.e. use putty n epoxe to cover. then sand to how thin u want the disc brake be. i think there're some pretty thin buttons available. so u can go try those. but be forewarned. i've never tried it. just thought abt it...