When asking a third person's willingness or asking a third person to do sth - is "would" more polite?

Question

A friend and I have decided to go on a picnic this weekend.

Can I use "would" to ask about a third person's willingness? Does "would" sound more polite than "will"?

Example 1

Would David go to the picnic with us?

Example 2

Will David go to the picnic with us?

How about when making requests?

Example 3

Would David go buy dinner for us?

Example 4

Will David go buy dinner for us?

Answer 1

In your examples, when you use would, you introduce doubt or uncertainty about your participation, not just David's. When you use will, you're limiting the doubt or uncertainty to David.

Will David go to the picnic with us?

This implies that you are going to the picnic. The uncertainty is with David.

Would David go to the picnic with us?

This suggests only that you are considering going to the picnic. Your decision might depend on whether David goes, or you could make an independent decision. Using would makes the entire activity uncertain.

Similarly:

Will David go buy dinner for us?

This implies that you are going to have dinner, whether or not David buys it.

Would David go buy dinner for us?

This suggests that you might not have dinner -- even if David agrees to buy. The use of would introduces doubt or uncertainty about the entire activity.