Aaaaand sanded again. Much smoother now. No ridges and plenty of feathering. Not sure if it needs a bit more taken off or whether the extra is helping to maintain the curve of the bumper. Will get another sand before priming anyway, so can deal with it then. Got a headache now though, not sure if it's from heat exhaustion in this 30°C weather or inhaling enough fibreglass dust to kill a horse, but it's nothing an ice cold beer can't fix either way.