Select to expand quote
Dar said.. Bit size should be 10mm to 20mm and at least 40mm deep. Even larger is recommended for deep cuts (finboxes etc).
If you're only working with soft foam, I don't think this is such a big issue, there's no where near as much pressure on the bit with soft foam as there is with timber. Just don't press too hard. I've used a 1/4" bit for ages on deep fin boxes without a problem. But if you're removing large amounts of foam a bigger bit is much quicker.