My experience for car model, for those teenee tiny part at the engine, which is not very exposed or for exhibit, you can just paint it or spray it (can spray) from the tray, then hand detail it, then only cut it out and glu it to the appropriate place.
As for big portion like the body, it is always better to cut it, extra fine sand paper it, wash, dry then only spray paint it. Hand brush on large surface usually not so nice, unless you mix the paint well (paint + solvent).
Also, when using spray can, always group parts of the same colour then spray it togather (save paint). and DON'T hurry, spray it thin layer, let it dry, then another thin layer, dry it and again... till you get the colour out.
Can spray from hardware store usually have larger grain size, so must spray it bit by bit, layer by layer, don't rush.

Should work just fine