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Using Polyester resin for repair below the waterline

Discussion in 'Marine' started by Chris U, Nov 1, 2016.

  1. Chris U

    Chris U New Member

    Hi there. I am hoping you can help with the dilemma I am having with a repair below the waterline?

    I purchased a boat and found that the keel strake had been grounded and the strake broken. This was then badly repaired and I had to rip out the whole deck.

    Where the keel strake was broken, there was a crack in the fibreglass and gel coat. There is now an area of about 200mm x 400mm that I need to remove and replace with new GRP as it is deformed. However, after reading various articles about GRP repairs, I am a bit confused about the best way to do this. I know that I cannot lay gel coat onto Epoxy, but I also read that Polyester is not good at bonding with older Polyester resin.

    Would I be best to:

    a) Remove the area that is damaged, bevel the area around on one side only, attach a backing plate, apply gel coat and then build up the area from the inside?

    b) Remove the damaged area, bevel both sides of the damage, apply a backing plate on the outer, build up the inside layers and then do the same to the outside, finishing the gel coat using PVA?

    If there is another way or suggested layup that you could recommend, it would be appreciated.

    I cannot really grind back a 12:1 area around the damage as the boat is not that long!! The damaged area is about 600mm forward of the transom.

    I intend to lay a new layer of Polyester resin with 450g CSM though out the entire boat and finish with topcoat. Would there be any benefit from backing the area with the repair with Epoxy after completing the Polyester/CSM layer?

    Sorry if this question is long winded, but I would rather not have this repair come apart when I am 5 miles off the coast in 150m of water!

    Thanks for your assistance.
  2. Martin @ EC Tech

    Martin @ EC Tech Tech Staff Member

    HI Chris, Epoxy is the best type of resin to use for under the waterline, it will give you the strongest repair. however you are right Gelcoat /flowcoat will not chemically adhere to it and polyester does not have the best mechanical bonding properties. Epoxy is way more superior for mechanical bonding so it is ideal for old laminates. you will need to paint the over the finished repair with an epoxy primer and boat paint or anti-foul rather than flowcoating.

    I can best advise on this if I can see the repair so please feel free to e-mail any pics to me at tech@ecfibreglasssupplies.co.uk Kind rgds Martin

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