Yuppy said..I mixed up a batch of epoxy resin and qcell. Smeared it over the board and wrapped it in glad wrap to keep it from running off.
Another lesson learned. Glad wrap creates a whole bunch of bubbles. Looks like I've got lots of sanding to do tomorrow. Fingers crossed.

question. Is it ok to sail now? At what stage is it waterproof?

OK so if you needed to hold it place, the mix wasn't thick enough, add more q-cell.
The qcell at this stage only needs to be a thin coat, your previously sanded board looked close to being fair enough. Spread it on evenly with a squeegee about .5mm thick, then all it needs is a light sand back.
Yes as long as you didn't sand through all the fibreglass it should now be ready to sail.