Fibreglass repair

[joeh]

dear panel
i am about to do a glass repair to a hull and skeg joint
my question is : after grinding, cleaning , drying the area (vertical )
should i apply layers of epoxy resin with suitable cloth first or gap fill with thickened epoxy first ? i am going to do both but unsure which procedure comes first.
ta

 

[john_morris_uk]

'Repair to a Hull and Skeg joint' isn't very specific, so a bit of a non specific answer:

The strength of the repair will come from the woven glass mat, so grind back lots, wet out the mat with epoxy (or wet it out as you aply it) and build up layers of mat. Maximum mat with the least amount of epoxy that you can get away with. The mat MUST go completely translucent as you wet it out.

When all that has hardened, fill and fair with epoxy with a suitable thickening agent. Its not hard to do. Its a little trickier to do it well.

 

[snowleopard]

It is perfectly satisfactory to use polyester resin. The idea that it will not bond to old grp is a myth. As it is underwater, use isophthalic resin, not the cheaper ortho.

Whatever resin you use, put the fabric on first or there will be a section of weaker material embedded.

Don't use mat with epoxy.

 

[MarkGrubb]

I've often wondered about this myth too, though I think what is normally said is that epoxy forms a much better bond with GRP than polyester resin. I've never heard it said that polyester resin won't bond to old GRP.

However, such handwaving statements are never much help (not yours, the general statements about the quality of the bond), and, naturally leads people, including me, to believe that the bond formed by polyester is inadequate for use on yachts. However, I've never been able to a definitive answer to the question: is the much better bond given by epoxy necessary? or is the polyester bond good enough?

Reading between the lines of your post, i'm guessing that you are suggesting that strength of the bond formed when repairing with polyester resin is more than adequate for yacht repairs, and to use epoxy will give a better bond, but is probably over engineering the solution. Is that correct?

 

[30boat]

I've done repairs to highly stressed areas on boats,things like strenghtening the bottom in the way of the keel and rebuilding the bow tip where the forestay attaches,using only polyester.Providing it is done properly I'd say the bond is just as good as with epoxy.

 

[oldsaltoz]

G'day Joe,

You must have a solid base to glass onto. Do not fill holes, grind them into craters or hollows that you can build up to a suitable surface.

After grinding and cleaning with acetone, start with small pre cut patches and build up the low areas first, then you can add larger patches.

When working below the waterline we only use epoxy resins and cloth designed for use with epoxy resin; it's important that the cloth is wet through, use a small threaded or grooved roller and roll out any and all excess resin. Leaving too much resin will weaken the job if using epoxy. Aim for 1:1 (One to one) ratio by weight of cloth and epoxy resin. Poly resins need 3 parts resin to one part mat or Chopped Strand Mat (CSM).

Mix resin in wide shallow containers, use latex surgical gloves, if you put 2 gloves on each hand it's easy to slip one off when it gets sticky. Fresh resin will wash off with warm water and soap, tools can be cleaned with white vinegar to remove the bulk, then final clean with acetone. Stand contaminated Acetone in a sealed glass jar overnight and reclaim the clean top layer for re-use. If your roller gets forgotten and resin cures on it, apply heat and the resin will burn off, wire brush it and ok to use again. Monitor temperature of layup after 5+ layers, resin may heat rapidly (Exothermic) and spoil a few hours work. All normal safety precautions apply, so get a good quality respirator, goggles and ear plugs/muffs.

Hope this helps.

Avagoodweekend......

 

[snowleopard]

Most of those who quote 'Epoxy is an adhesive, polyester is not' and such things are mainly going from hearsay. My boat is built from panels which are bonded together with glass tape and polyester resin. The designer has 40 years of experience with these techniques and has done plenty of experimentation. He finds that a polyester to polyester bond has at least 70% of the strength of a one-piece moulding, and epoxy on polyester about 10% higher. (Polyester onto Epoxy is a no-no).

After nearly 20,000 miles logged, I have seen no trace of de-lamination of joins so I'm inclined to believe him.

I did some experimentation myself to test the criteria for a good bond. I took a moulded surface and treated it in the following ways:

1. Untreated, as it came out ofthe mould
2. De-grease with Acetone
3. Abrade using 24-grit disc in angle grinder
4. Abrade then de-grease

then bonded a patch of glass on with polyester laminating resin.

Tests 1&2 resulted in the patch cracking off easily. With tests 3&4 I could not break the join, the laminate itself failed first.