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filling a GRP hull

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Hi i have a thick GRP hull 1970's birchwood

there are lots of old through hull fittings which are basically in the way or in the wrong place and some i've drilled and then thought oops :oops: lol

so i've now took the old girl out of water to sort them out

what is the best way to sort the holes out, what i was going to do was

fibreglass a patch from the inside after it is cleaned properly :)

then fill with polyester body filler from the outside to blend it in with the rest of the boat

there's no gel coat to worry about as it's a rough old boat anyway and i'm now going to sand her all down and paint and polish anyway

i think the gel coat is under layers of paint and fibreglass anyway lol

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The generally accepted method with GRP is to countersink the holes from both sides, to expose clean bare laminate, and give a much larger contact surface for the repair.

Then use Epoxy resin, not Polyester, because it is far more adhesive to old cured GRP and is much more impervious to water when cured.

Whenever I've done it, as per these pictures, I've applied two layers of woven mat cut into discs and wetted that out with epoxy resin into the craters on both sides of the holes, giving four layers of cloth in total. I waited just long enough for each layer to "gel" then applied the next, while the epoxy was still green. (otherwise you have to sand it down to get a key). Then, finally, I filled the hole level to just proud of the hull surface with epoxy mixed with colloidal silica (to stop it sagging whilst upside down). By the next day it has gone rock hard, and can be machine sanded down to a perfectly flat surface level with the hull.


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oh right kool so the epoxy resin is strong enough to actually fill the hole?

obviously with matting to bridge the gap and put as much matting in the hole as possible :)

it doesn't need to be too cosmetic because none of the holes will be seen

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oh right kool so the epoxy resin is strong enough to actually fill the hole?

I quite understand your fears there. The thought of the consequences should one "pop out" are quite frightening. :shock:

I had five holes to block off on my current boat, and I searched in vain for a stainless or bronze "blank" skin fitting, because I fancied the idea of something made out of solid metal, rather than wondering about resin adhesion and strength.

I made extensive searches through Google, most of which ended up at the very active YBW forums. The overwhelming consensus there was that it was better to do a permanent filling job, rather than blank them off. You could use a new DZR skin fitting with either a female blanking cap, or a seacock with the handle removed, but almost everyone thought that was a bodge, unless the hole might be needed at some time in the future.

To give myself more confidence with epoxy, I tried a test piece where I spread a blob of it over a piece of fibreglass cut from my boat when a hatch was fitted. I sanded through to clean laminate, wiped it with acetone, then spread the epoxy on. After a week I tried to chisel it off, but it had stuck so firmly that the only way was to machine it off. I even made a saw cut between the two materials then tried to lever it apart, but it wouldn't budge. :)

As for tensile strength, the four layers of mat, applied in one go through both sides of the chamfered hole makes a sort of rivet, so even if the bond did break, it couldn't move out or in.

As long as the GRP is cut back carefully and is clean, and the in-date epoxy resin is mixed accurately to the right ratio, it goes of like a rock.

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i've just found my box of fastglass filler stuff and have order some caboldy stuff off flebay

For underwater repairs and hole filling, I wouldn't use anything else other than genuine epoxy (liquid) resin, woven mat, and approved epoxy filler additives, (like colloidal silica).

Although most resin car body (paste like) fillers are OK for above the waterline, I would wonder about subsequent water absorption with anything other than epoxy.

Epoxy resin is available in two pack can sets, like "West Epoxy", with either fast or slow cure hardener. The mixing ratio is critical, unlike with polyester resin, where the catalyst works differently. I use a cheap set of kitchen electronic scales and disposable plastic party beakers and spoons. That way, you can mix up very small quantities very accurately, (like 20 grams), so wastage is kept to a minimum. It only stays workable for about 15 minutes in the Summer.

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Hi Strow, in your post above, you mention "fast and slow cure hardeners". Is there any significant difference, in each of them, such as the slower cure may be stronger and the faster cure may be harder, but more "brittle" if you can understand what i`m getting at?. Regards ................. Neil.

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