Polyester resin to fill holes in hull

[Kyle2]

I am fitting a new engine to my 1974 Snapdragon. The water inlet and exhaust outlet require larger holes than the old engine, so I have to fill the holes left behind. There is a 30mm hole in the transom at the water line, and three small holes (about 8mm) around the main water inlet hole. I have used polyester resin and glass fibre mat before for fixing things to the inside of the hull, but I think I need to be more careful now to fill holes where water might come in if there is anything wrong with my repairs! This is my plan - does it seem a sound one? I'll chamfer all the holes on the outside. The large hole I will then abrade the GRP for 4" around on the inside and add layers of mat and resin to a thickness of about a quarter of an inch. When that has set, then I'll add resin and glass strands chopped small to fill up the hole on the outside. Finally, when all is set, finish off the outside with gelcoat filler. The small holes I'll block with cling film on the outside and fill with resin from the inside. When set, smooth off with gelcoat filler. Comments on materials and/or method gratefully received.

 

[bigwow]

epoxy resin, rather than polyester, would give a stronger repair

 

[Kyle2]

I think I read somewhere that polyester resin bonds better than epoxy to the original hull, which is also made of polyester resin.

 

[nimbusgb]

Which ever resin you decide on ( I would rather go epoxy ) The real secret is to chamfer out both the outer and inner hull areas around the holes over an area of at least 1:10. Cut several rings of cloth of increasing size to suit your patch and lay them up from the outside, closing off the hole. Then move inside and fill the majority of small diameter holes with a resin mixture thickened with glass balloons to the point where you can repeat the same as the outside job.

The idea is to form a sort of yoyo effect with the inner and outer 'mushrooms' bigger than the hole and joined by fresh material through the old hole. You need to fill small holes because you simply can't lay glass into anything less than about 25mm effectively.

With a bigger hole such as left by a skin fitting then chamfering off can be done to such an extent that the outer and inner laminations meet and form a really unitary structure. A patched 50mm skin fitting may have a last layer lamination 500 mm across. That may sound excessive but to retain the integrity of the hull may be necessary; it also avoids weakening the patch when fairing and regelling or painting is done.

 

[lw395]

Sounds absolutely fine. If you abrade the grp and clean it with a bit of acetone, the polyester will stick very well, provided it's done at a reasonable temperature.
Use a smooth board covered with ordinary polythene bag as a mould on the outside. When the structure is hard, sand it down a little and finish off with gel coat. Real gel coat seems to be a lot better than the gel coat filler sold by plastic padding imho.
Epoxy sticks well to polyester, but its more expensive, and the gel coat does not stick so well to the epoxy, although it's fine for a cosmetic coating.
CFS in cornwall are good suppliers.

 

[swiftnsure]

The method of filling the holes seems fine in itself. bear in mind that epoxy will stick well to the original polyester, but polyester doesn't like to bond to epoxy if you should need to at a later date. what I cannot understand is why you don't just bore out the original holes to fit the larger skin fittings of the new engine?
I, personally, don't like too many holes in my boat /forums/images/graemlins/wink.gif

 

[charles_reed]

You can get

glass fibre filled polyester, which will do the job perfectly.

I've used it both above and below the waterline. Just make sure you use a former on each side.

 

[leomagill]

Re: You can get

You can get a better bond by wiping the prepared area with styrene which partially re-activates the original resin giving a chemical bond as well as the mechanical bond.

 

[Kyle2]

Thanks to all for your help. I decided to fill the old exhaust outlet because the new hole at 50mm seems rather large to have at the waterline. It would let the water in very fast, if there was any problem with the exhaust or skin fitting. Maybe this is an unreasonable fear? The new 50mm hole will be approx 6" above the waterline.

 

[cliff]

The 1:10 rule of thumb applies to the thickness of the hull at the hole NOT the hole dia. I.E. 10mm thick - 1:10 taper is 50mm (you feather both sides so 5X10 is the length of the feather/taper not 10X10)
---------------------

Artificial Intelligence is no match for Natural Stupidity

 

[Clyde_Wanderer]

[ QUOTE ]
I think I read somewhere that polyester resin bonds better than epoxy to the original hull, which is also made of polyester resin.

[/ QUOTE ]

You have it the wrong way round.
Epoxy bonds better than polyester.
You can not use chopped strand mat with epoxy, as epoxy wont disolve the binders in the mat, so you will need to use a woven roven or biaxial cloth, then you will have the problem of that polyester gelcoat wont adhere to the epoxy,afaik.

 

[PCUK]

West epoxy have brought out a new chopped filler mix exactly for this purpose that also bonds well with gelcoat.
Wessex resions for details.

 

[tangomoon]

[ QUOTE ]
epoxy resin, rather than polyester, would give a stronger repair

[/ QUOTE ]

Agree unequivicobaly

 

[snowleopard]

Two important points to make here.

(a) you can make a perfectly satisfactory bond with polyester resin. My boat is built by bonding panels of GRP together using polyester for the joins. The designer has done extensive research on the subject and I have done my own tests where I found failure occurred first within the laminate rather than along the join. I can state with complete confidence that a polyester to polyester join is as strong as epoxy to polyester (and a LOT stronger than polyester to epoxy). I have effectively bet my life on it! In any case neither will stick unless the surface is thoroughly abraded. De-greasing with solvent is NOT enough.

(b) When using polyester resin for below-waterline work it is essential to use isophthalic resin, not the normal orthophthalic type. If you just buy 'laminating resin' off the shelf you will get the wrong stuff. Ortho is the stuff used on early GRP boats which caused all the osmosis. Iso is around 30% more expensive.