Read the directions printed on the can.
Why would you?
Yes, you can.
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.
Yes, you can.
It's a very good marine quality varnish.
Only if it is a car with exposed wood. Called a Woody. If you are painting metal the results will be unsatisfactory.
Look at label on can if urethane is oil based it should be fine. Think of it like basic chemistry. Oil and water dont mix, xlene based products are used to strip oil, so if ur using a xlene base over oil thats a no brainer. The urethane I use is a xlene base so I wld never use over oil. Im just not sure if they sell a oilbased urethane so check the labels
You can use spar, however I do not recommend it. In my experience spar tends to yellow over time and that would alter the artwork. Also, if you are planning on using the bow as anything other than a decoration hanging on a wall, I'd check with a good bowyer to be certain the top coating you intend to use won't crack, check or in some way limit the usefulness of your bow.
Yes, but why waste the varnish. - (it is meant for bare wood )
Yes, I just did it last week. If it's for outdoors, make sure to put 2 to 3 coats of poly on it. I recommend Minwax Helsman Spar Urethane.
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.
I would suggest using spar urethane as it protects fromm moisture,sun light and temperature changes. Follow drying times as recommended on can. Three coat minimum maybe more.