I agree with everything above, having sprayed quite a few cars in the past (ie, cost of all the extras, etc. and the higher cost of materials, toxicity.....) Given all that, if you are still intent on EITHER brushing, or rolling on the paint, I would highly recommend using the 3-part Awl-Grip marine epoxy paint. The three parts consist of a) pigment, b) hardener, c) reducer (specific for either spray or brush/roller). The pkg. is relatively expensive for those three materials (but 2qts of pigment will do a whole LBC with touch-up left over), but will require much less wet-sanding, finishing etc. for a super-high-gloss finish and will provide a super hard, solvent/sap/road dirt resistant, compoundable, buffable and/or polishable surface allowing for easier "spot repairs" later. A few "thin" coats are absolutely required for a final finish, and a little practice in application is needed (including the art of "tipping" the paint). Good Ventilation is an absolute must, so "basement" jobs are out of the question!
I am doing mine this way and have experimented on one door and a mirror, just with a brush and the results are great!