That should work its not as good as using a acid etching primer you might want to test the bond to the bare metal since you already tested the primer on bare metal take some good 2" masking tape I the stickier the better and rub it onto the dried primer so its bonded to the primer very well then rip it off quickly and see if any of the primer come off the bare metal that will give you an idea how well its bonded to the metal. If none of the primer come off you should be good to go also with a spay can primer like that your going to have to do thinner coats so instead of 2 to 3 coats it will be more like 5 to 6 coats and then you should be able to sand the primer smooth with some 400-600 grit sand paper and apply your flat black after 3-4 of coats let it dry well then take some 800-1000 grit wet dry paper and lightly wet sand the surface again to remove any imperfections dry it then clean the area and tack and apply 1-2 final coats let them dry and you should be good. Since your using flat black you can not sand any on the final surface since you can not rub out any sand scratches so you need to do your final sanding prior to applying the last couple of coats.