The W-8 hasn't arrived yet but I need to pay my foreign payee now — should I just over-withhold at 30%?

The clock runs off the payment, not off when the form arrives. Publication 515 says a payment is made when the person realizes the income, whether or not cash changes hands, and it can be made for the person's benefit or to their agent. So you cannot stop the obligation simply by delaying the form. If you are going to release the payment, the withholding analysis has to happen at that moment.

When you do not have valid documentation, the IRS presumption rules generally direct you to treat the payee conservatively and withhold at 30% under Chapter 3 or Chapter 4 as applicable. The requester instructions warn that if you fail to obtain a valid W-8 or W-9 and fail to withhold under the presumption rules, the IRS may assess 30% plus interest and penalties — and remember from section 1461 that you, the agent, are personally liable. Between over-withholding and under-withholding, over-withholding is the recoverable mistake: the payee can claim any excess back on a US return supported by the 1042-S.

Withholding at the presumption rate is a defensible stopgap, not a substitute for getting the right form on file. Keep your source analysis, your attempt to collect the W-8, and your rate computation in the file, then obtain the valid documentation so future payments can use the correct rate. Our /guides/form-1042-s overview covers the presumption rules and how to paper the file when a form is late.