I just sealed a couple of cork floats with POR-15 Fuel Tank Sealer. Got a pint for under $20 including shipping at Amazon. I’ve been reading around about it’s use for sealing cork floats and though I’d give it a try. Seems to go on very thin and has a water like consistency. A RD-19 cork float will fit into the can.
http://www.por15.com/POR-15-Fuel-Tank-Sealer_p_64.html
