Only if it is a car with exposed wood. Called a Woody.
If you are painting metal the results will be unsatisfactory.
Yes, you can.
Yes, you can.
Why would you?
Assuming you're talking about wood finishing, spar urethane can be used over a water sealer. In fact, the combination is better than just a sealant or just the spar urethane. A very durable finish for outdoor wood products is soaking the unfinished wood with boiled linseed oil, letting the linseed oil cure, removing excess cured linseed oil, then applying spar urethane.
It's a very good marine quality varnish.
Only if the urethane paint is not waterbased... Urethane Paint cannot be mixed with anything except other Urethane products. Urethane Paints do not work on the same principals as "oil based" paints. Oil Based Paints rely on evaporation to dry. Urethane cures. by chemical reaction.
Yes, that was the original intention of urethane.
I use Urethane, Matrix paint is the namebrand I use but there are plenty out there, Dupont, Basf, PPG, and Sikkins
Thinner--specifically, mineral spirits. Reducer is used in urethane car paint.
Yes, you can use urethane paint on porcelain mugs.
I guess it depends on what type of project you are doing. if it's something that is going to be outside all the time or most of the time use an exterior paint for best results.
Read the directions printed on the can.