Hey, Brother.
Welcome to the forum. You'll find a lot of very helpful people here.
I've re-finished many 1/18 diecast models - and many AutoArt (often called Biante) cars.
Firstly, disassemble the model - I carefully take off ALL the plastic bits from the metal body, noting which screws go where (many are different sizes). The cars go together, so they have to come apart. Just try not to force anything.
When I've got the car apart, I strip the paint using paint stripper. If you've left any plastic bits on, the paint stripper will ruin them.
When clean, I inspect the body for flaws. I usually rub the body with 400-600 grit to help paint adhesion. The paints I use are automotive acrylic (use a correct filtered face mask when you paint) from a touch-up spray can - I use a short length of drinking straw to fill my airbrush bottle - place the straw over the nozzle and shoot the paint into your bottle - and spray everything with my airbrush. Use a good quality paint. I spray an automotive etch primer (it really is necessary) onto the metal and fix any flaws with spot putty. This etch primer adheres to the bare metal better than normal primer.
Then I spray a coat of normel automotive primer, rub a little with 600 grit if needed, re-prime (if necessary) then spray the color coat from my airbrush.
You can experiment a little with the pressures and thinner ratios, but I spray at 10-15psi with the paint fairly thin - this prevents the dreaded "orange peel", and I can usually get an extremely good gloss right "off the gun" - with no need to "cut and polish" after spraying.
Youll notice that the original paint was very thick and it sometimes hides surface detail - you'll end up with much thinner paint when you're done.
The good part about using automotive paints is they dry very quickly, dry pretty hard (but can chip fairly easily) and can be polished with polishes used on real cars.
There should be no need for a sealer, unless decals are to go over the paint.
Just be careful when reassembling the car - it's easy to chip the paint if you're not careful.
Good luck!!