How hard is it to repair GRP?

[crisjones]

Generally some pretty good advice from everyone. I would say polyester resin is perfectly adequate for the repairs you are looking at, no need at all for the extra cost and hassle of epoxy.

Grind back the damaged areas inside and out until you get to original, undamaged lay up and then build up from there. Use something on the outside to give some shape (does not need to be perfect) while you start the repair on the inside, this only needs to be stiff card with a plastic bag on the inside - tape it on the hull and put some layers on the inside.

Best to only do 2 -4 layers at a time, wait for it to go off (4-6 hrs) and then add more layers if required. Never be afraid to put too much on - it is easy to grind off!!! Once the inside is done, remove the support and then grind all over the new stuff, the dodgy bits where it joins the existing and anywhere else that needs it. Then repeat the lay up process on the outside, again do not worry about putting too much on - much better than too little. Once this has hardened you can grind / sand it to shape as required before adding a gel coat layer that is also then sanded back as required. Finally finish with paint to match colour.

The best piece of advice I can give is this : it is virtually impossible to repair grp without grinding - before, during and after!!!! Resin and matt are relatively cheap so put plenty on and grind back after, you will get a much stronger job than trying to put the bare minimum on to give the required shape. It is also much easier to produce the required profile with a grinder and sander than with resin, mat and roller.

 

[William_H]

F/G repair

I also would use polyester resin. It is easier to make a mix which will harden quickly at your temperature.
You will likely end up doing quite a few lay ups. Layup, grind, layup, grind etc. Polyester does not stick to polyester as well as epoxy but if the mechanical key is good then no problem. Polyester will stick very well to new polyester ie within a few days of layup. So if you put a final patch on the inside that overlaps the repair and good f/g you will get a good key bond.
Use fibreglass cloth or chopped strand mat to build up the bulk and make it stronger.
Polyester can be a devil to grind if it is not hard. It can seem quite firm but is best shaped with a knife or a plane rather than sand paper as the sandpaper will clog very quickly.
Do small batches of resin with a fair bit of hardener in and be prepared to throw the batch away as soon as it thickens. As said try to get the job and the area warm. good luck olewill

 

[Philthy]

Thanks all.
I'm not sure about the tent idea, but will consider it. The irony is, it's perfect today, nice warm bright sun!
I think my plan of attack, will be to remove the bowplate cover hatch in the fore cabin, and start there, then build up from the outside.

 

[Fantasie 19]

Thanks all.
I'm not sure about the tent idea, but will consider it. The irony is, it's perfect today, nice warm bright sun!
I think my plan of attack, will be to remove the bowplate cover hatch in the fore cabin, and start there, then build up from the outside.

Would you put up pictures as you proceed?? Would be really good to see step by step...

 

[Philthy]

Would you put up pictures as you proceed?? Would be really good to see step by step...

Will do.

 

[fisherman]

When laying up use no more than 3x600gm mat at a time, or it will overheat. It is easier to wet out the mat on a board, then brush on a layer of resin before applying the wetted mat. That way it does not matter if the previous layer is not too smooth, push the wetted mat on with a brush. It is not good to use excess hardener (catalyst) it can make the job brittle, so no more than 2%. Better to use gentle warmth from a lamp. (Your resin will probably be pre-accelerated, but extra accelerator is the answer if it is a bit cold: a GRP car panel man near me has a thermometer on the wall and a chart of accelerator and catalyst amounts, you need not be so finicky, he is cutting overheads.)
As soon as the mix in the pot goes like marmalade you have a bit of time to work on the job, which is slower because it is thinner. As above you will get a mechanical bond on old GRP, and a chemical one on recent work, for a few months, I think.
Don't panic, it's really quite easy.

 

[Forbsie]

If you have a video player, PM me your address and I'll send you the West Systems video - "Fibreglass Repair".

 

[basil421]

Make an outside mould

If I were doing this repair I would make a mould before grinding anything away using the following technique.

Repair the hole with plaster of paris. Its quick cheap and easy to work. When happy with the shape paint it using a cross linking paint of varnish. When cured apply liberal PVA (that's poly vinyl ancohol) release agent then lay up a mould on the outside extending a good 100mm beyond the damaged area in all directions. When cured remove the mould, break away the plaster and proceed with the necessary grinding as described by others in this thread.

When laying up the repair you will have a mould to give you the exact shape of the outside instead of having to grind and shape grp which believe me as no easy job. (Remember to use PVA release agent on the new mould as well). Your time spent making this simple mould will be repaid and the result will be a superior shape and finish.

I would use polyester for both the mould and repair as long as the bond area on the inside of the hull is at least 100mm wide and has been ground back into undamaged GRP.

 

[Boo2]

If you have a video player, PM me your address and I'll send you the West Systems video - "Fibreglass Repair".

Or it's on youtube in two parts here and here.

Boo2

 

[nickcirrus]

I use melamine faced hard board, or just a plain sheet of hard board on which you you can apply some mould release (either PVA or wax). Then Fix an appropriate sized piece of the board to the outside of the hull to pick up the hull shape across the hole. Gaffer tape should hold it in place or props to the garage wall or self tappers. To help it to form around the composite curves of the hull some cuts to the board on the back will help or spray some water to the back to dampen. This will give you a nice mould on which you can apply two layers of gel coat from the inside then when that has cured lay in the mat. Good feathering of the glass on the inside up to at least 6" all round the hole will give a satisfactory bond. When all is cured, Peel off the hard board and fill any voids in the gel coat.
Polyester to polyester will always achieve 90% bond strength as long as it has been cut back with 60 grit sandpaper. Mini grinder with sanding disc is best.
Time spent setting up mould will result in less finishing on the outside! Tearing the glass into bite sized pieces first and then wetting them out on a board with a fluffy roller before applying and taking care to overlap will keep things from getting out of hand. Get the glass on quickly then use a metal roller to remove air bubbles.

Good luck.