Damaged fibreglass hul

[Bigbang]

Hello I had an incident where the rollers flips 90 degrees outwards when launching and the boat hit the pin twice causing two holes, I’ve seen on line I can repair with some chopped strand mat after cutting and sanding down, but I’m unsure what I should use as a final coat to seal, will using the resin as a final seal and then gel coating be enough? It’s below the waterline

 

[Tranona]

Welcome to the forum

Repairs to GRP are indeed possible, but perhaps not as simple as you describe. Suggest you look on www.wessex-resins.com where you will find information on repairing GRP using epoxy which is generally preferred for repairs such as you are contemplating

 

[Assassin]

Repairing GRP is easy but you need to master some basic skills which apply to both GRP and epoxy based systems, first you need to establish the extent of the damage as both GRP and epoxy get a condition called stress fractures and delamination and you need to establish their extent.

For small holes:

Cut away the damage and cover the hole with some heavy duty plastic sheet such as heavy polythene and ensure this has a release agent liberally applied to the face which is facing the hull, cover this with a stiff tape and ensure the polythene is taught and follows the correct lines of the hull, you can stiffen this by adding some layers of 2mm model making ply to the outside of this to stiffen the polythene and ensure it is the correct shape.

Mix a little gel coat and paint only the polythene layer from inside the hull, this never goes off so let it go very tacky and apply another coat and let this go tacky.

Now you need the resin and matting, never use CSM as this will show through the finished repair, only use a type of CSM called tissue as this is much finer, cut a piece to fit your finished hole, paint a layer of resin into your hole and apply the tissue, apply another layer of resin over this and stipple it to ensure all of the tissue has absorbed the resin, and let it harden. Repeat this with a second layer of tissue.

Now you prepare your hole edges inside the hull and grind them back to a 60 degree angle using a diamond tip cutter and have a vacuum in close proximity to collect the dust and to keep it as clean as possible, and abrade the area around the repair to at least 50% of the diameter of the hole.

Wipe the finished area with a paper towel coated with a little acetone and by coated I mean just a little and wipe the area just enough to remove all the dust and clean it up, let it evaporate.

Cut some CSM and build the area up in the hole until it is reasonably level with the surrounding area and then cut progressively larger pieces of CSM and bond them in until you have several progressively larger pieces going into the prepared area, and until the prepared area is fully covered, leave overnight to fully cure.

Safety - always wear a respirator when cutting or grinding GRP or epoxy, always wear disposable gloves and double glove when applying or mixing GRP or epoxy as it is a bast*rd to get off skin and clothing.

Once fully cured, remove the hole covering and check the finish, use some fine grinding paste or fine abrasive polish and cut the gel coat to finish it, but don't cut through it, I like Auto Glym paint reconditioner as this gives an excellent finish on gel coat, then polish and anti foul if necessary.

The only difference between GRP and epoxy systems are the materials used, GRP uses a polyester resin and a glass mat whereas epoxy uses an epoxy resin and carbon mat.

 

[TQA]

The only difference between GRP and epoxy systems are the materials used, GRP uses a polyester resin and a glass mat whereas epoxy uses an epoxy resin and carbon mat.

Using carbon mat on a repair would be unusual generally regular glass mat or cloth is used. But there is a GOTCHA regular glass mat is often made with a binder that does not dissolve in epoxy resin. Just make sure that you explain to your supplier that it is for epoxy. NB Better to go to a specialist and not Halfords.

 

[Assassin]

Correct, this is why I stated polyester and glass mat and epoxy and carbon mat and didn't delve deeply into it for a beginner to composite repairs.