When glass reinforced plastic (GRP) first hit the boatbuilding scene in the 1960s, it was seen as something of a miracle. ‘No maintenance required,’ screamed the advertising. ‘A glassfibre hull will last forever.’

Alas, Mother Nature had other ideas, and gave us the problem that can drain an owner’s face – and wallet – upon discovery. In a word: osmosis.

The real culprit is the hull’s outer decorative layer, the gel coat, which isn’t entirely impervious to water, especially when coloured with a pigment.

Thankfully there are products on the market that will create an impermeable barrier, so once the blisters have been drained or better still, before any have even formed, seal your hull using epoxy.

For the full feature buy the May issue of Practical Boat Owner, but here you can see Jake Kavanagh’s step-by-step demonstration of how to prepare your hull for a coat of Gelshield