How hard is it to repair GRP?

[Philthy]

Msida broke her mooring a few weeks ago.
In short, she is going to need some GRP work. The worst bit, is the bow, which ground itself away. I have access to all the necessary tools and safety gear, but would like some pointers about just how best to repair her? The rails are not a problem at all.
Thanks in advance.

 

[Spuddy]

I've done some repairs above and below waterline. Easiest when it's possible to patch up from the inside - chamfer off the holes to 1 in 6 (or was it 1 in 12), blank off the outside then build up layers. I concluded with a poultice of stuff overlapping the inside skin. This was where appearance not important. Getting a faultless match with gel coat on the outer skin is altogether more involved and skilful.
Oldsaltoz is the mastermind on this sort of thing - if he turns up, I'd follow his advice.

 

[Philthy]

She's painted, so my main concern is strength not looks.
Fortunately, I can get at the inside of the hull where necessary.

 

[Boathook]

You will possibly have to cut back quite a bit of the old GRP to get to the undamaged bits before you start to actually rebuild. The problem is then trying to match the hull shape / line. If possible do the repair under shelter. Also wait for other posters ....

 

[Philthy]

You will possibly have to cut back quite a bit of the old GRP to get to the undamaged bits before you start to actually rebuild. The problem is then trying to match the hull shape / line. If possible do the repair under shelter. Also wait for other posters ....

She is sat on a trailer not ten yards from whee I sit now, and can simply be wheeled into my brothers garage when necessary. We have all the necessary air tools finishing equipment I think, so that's a weight off my mind.

I also have he tools to cut the affected areas out, but I suppose my concern is, would a "plug" type of repair bond to the existing GRP with enough strength? Thinking out loud, feathering the edges, then building up is going to be much stronger I would have thought?
Also, is it a problem to over build the repair, then sand/grind it back to shape?

 

[fisherman]

On a flat surface it is usually possible to pass a piece of ply in through the hole and screw from the outside, for a base. I've seen a repair to a stem done by pushing a plastic sandbag against the hole from inside, poke it about until the shape is good then repair outside, remove the bag and repair inside.

 

[gjgm]

West System provide some good booklets and videos; try Wessex Chemicals. lots of cunning tips.
Obviously the techniques apply to polyester resins too, but you need a bit of warmth, and very little humidity.

 

[fisherman]

Thinking out loud, feathering the edges, then building up is going to be much stronger I would have thought?
Also, is it a problem to over build the repair, then sand/grind it back to shape?

Feather the edges in and out, run the repair over the edges inside and out, it will be very strong. You can reinforce the inside as much as you like. Avoid grinding, for my money. Just hate the dust.

 

[Deleted member 36384]

.... would like some pointers about just how best to repair her? ....

West System provide some good booklets and videos ...

http://www.westsystem.com/ss/use-guides/

The above link is to West's guidance notes. I have very little experience with GRP repairs but have followed the advice in these guides to fill 6" holes and repair cracks in GRP. I recommend them as a sound technical resource written for the amateur.

 

[VicS]

In addition to the good advice you will find on Wessex Resins website you might find this old book produced by Tylers useful
http://www.topcatsail.co.uk/GRP-REPAIR-MANUAL.pdf

The 5/- price tag rather gives away its age!

A good way of dealing with large areas of damage is to find a similar boat and make a mould from that of the damaged section. You can then make a new "chunk" to go in to replace the damaged bit.

Polyester resin does not adhere well to old lay ups. Better therefore to use epoxy.
Remember resin and hardener proportions must be measured accurately if using epoxy .

 

[Philthy]

Some very useful advice there.
Thanks

 

[davidej]

In case it is not obvious, West and wessex systems are epoxy rather than the polyester resin your boat is made of.

many would argue that an epoxy repair would be stronger but IMHO polyester will be absolutely fine and much cheaper if done properly.

 

[fisherman]

And whatever you do don't get involved with epoxy dust.

 

[Bosun Higgs]

Msida broke her mooring a few weeks ago.
In short, she is going to need some GRP work. The worst bit, is the bow, which ground itself away. I have access to all the necessary tools and safety gear, but would like some pointers about just how best to repair her? The rails are not a problem at all.
Thanks in advance.

The sales director at Prout once told me that grp was infinitely repairable providing only that you dont set it on fire. And thats been my experience in practise.

 

[Philthy]

The sales director at Prout once told me that grp was infinitely repairable providing only that you dont set it on fire. And thats been my experience in practise.

Excellent !

I'd heard the dust is very nasty,so will have all the necessary masks etc.

 

[Lakesailor]

Choose your time carefully. Epoxy really doesn't like low temperatures. Or humidity.

Much nicer if you can get it in the garage and get the place heated.
Air tools tend to encourage you to lose the profile of the repair. That is, go too flat.
Longboard sanding is harder but you can work at the profile, comparing one side with the other as you go along.
I would tend to use polyester for your repair. Easier to work with and that area doesn't look strength-critical. If that damage on the top of the stem is all there is to repair it should be a doddle. I would also build up to no more than the finished profile and use marine filler on top of the grp to get the final shape. That sands much more easily and you will get a much fairer result.

 

[Philthy]

I need to make sure the bow plate has somewhere sturdy to be fixed, so will be reinforcing as necessary.
I has wanted to get it done soon, but if temperature and humidity are an issue, then it might have to wait. The garage is massive, so heating it sufficiently,may be a bigger problem!

 

[knuterikt]

I need to make sure the bow plate has somewhere sturdy to be fixed, so will be reinforcing as necessary.
I has wanted to get it done soon, but if temperature and humidity are an issue, then it might have to wait. The garage is massive, so heating it sufficiently,may be a bigger problem!

Build a "tent" inside the garage - less space to heat..

 

[Spuddy]

Temperature and humidity are more critical with epoxy but the stuff does stick better to existing polyester. Perhaps cobble up a tent around the bow with plastic sheeting and create a micri-climate inside.
It's really not difficult; basic grp lay-up is only a semiskilled occupation. Have a read up on the leaflets and books, use some common-sense and it all should go OK. Good luck

 

[wot]

What about a shaped wooden hardwood plug to go inside the hull? It could be shaped to fit and bedded in place using thickened epoxy. That would supply and satisfy any strength requirements as well as giving a base onto which to lay the grp.
The remaining work would be to restore the bow and stem profiles.