Try cardboard, or wrap your cardboard in wax paper. The paint doesn't like to stick to the wax paper as fast as it will cardboard or newspaper. (Been there, done that many times before.)
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When you're painting, get a piece of sheet metal to use as a test piece. (Anything will do, really. I use an old metal shelf from a unit we took apart in the garage.)
Spray your primer on the test sheet first, in about a 6x6 inch square. Then prime the tractor parts.
Use the test piece to tell if the primer is dry. Don't touch the tractor parts - if its still wet, you'll leave fingerprints.
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When you're ready to apply the paint, on the test sheet paint over about half of your primer and wait a few minutes. If it doesn't crackle or bubble, you're good to go. If it crackles on the test sheet, wait some more, and try it again. When it doesn't bubble or crack, you're good to apply the second coat. Keep going for each coat you use.
If its going on the tractor, its going on the test sheet first. That way, if it goes wrong, you've just effected the test sheet - no sanding of tractor parts.