Damaged Gelcoat below waterline on Westerly Centaur

[Refueler]

I have seen 2 Centaurs that had been fibreglassed straight over old oily filthy danboline bilge paint, nothing is going to stick to that! If you grind back to clean glass Poyester will stick perfectly, or Vinylester which approaches epoxy in performance but with ease of use of polyester.
One thing people who advocate epoxy often dont realise is that the glass reinforcement should have an epoxy compatible size (adhesion promoter), Glass intended for poly or vinylester has a different size.

The gauge or as usually stated the weight per area of glass matt is decided on the job it is to do - not the resin to use ... and having done a lot of glassing and had instruction from Strand Glass ... as long as the resin is of the correct fluidity - the matt will wet out.

With Polyester of course one of the biggest hurdles is the working time ...... so you need to work with thin matt to ensure wetting out before resin becomes unworkable.
Epoxy can be of course in far longer working time form ......

 

[Graham376]

had instruction from Strand Glass ...

A pity Strand Glass disappeared, those workshops/classes were very useful, did a couple in the 70s when I was working on grp cars.

 

[Javelin]

I've come to this thread a little late but having read some of the answers I can't help myself.
That said I'll try and address the op's question.


This pretty much applies above and below the water.
Cracking gelcoat is either down to impact or flex or both.
1st job is to eliminate the flex. (ie do your keel bolts up and try not to hit anything)

Flex cracking in gelcoat STARTS at the laminate and works its way outwards.
Impact cracking is obviously the reverse.
Either way when repairing gelcoat you have to go back to the laminate by grinding, sanding Dremel etc.
However below the waterline aesthetics are not important and structural integrity is.

Therefore the sensible thing to do is use epoxy resin. SP, West etc.
Epoxy will give a chemical and a physical bond to old laminate. It’s a very good adhesive.

Polyester will only provide a physical bond. It is a very poor adhesive.
(You can get a good chemical bond but only if the polyester has not fully cured, (You can still smell the styrene))

I’d need to see photos of the actual damage but my guess is,
After grinding and cleaning off, I’d screed over any indentations with west epoxy mixed with 403 microfibres to a consistency of thick mayonnaise.
The go home and come back the next day.
The surface will have a blush which needs removing so you can scrub this off with warm soapy water or just remove all the shine using 40 grit paper.
Remove dust and wipe down with acetone.
Assuming a flatish surface I’d then apply 4 to 6 layers of 250g biaxial.
Unless you’ve managed to turn her on her side (I’d like to see that !) you will be glassing upside down.
The trick here is to paint neat epoxy to the damaged area first.
Then on a piece of scrap plywood or even cardboard on the floor wet out the layer of glass.
Not too much but just enough that you have a 80% covering of epoxy.
You don’t want it too heavy as gravity is not your friend.
Peel the wet cloth off the ply/cardboard and stick it to the hull using a wideish brush and your now sticky gloved fingers.
As you’ve already wetted out the damaged area it will be easy to fully wet out without having half a litre of epoxy dripping down the brush and up your arm.
(you can make a little masking tape dam round the brush handle which will catch some)
I’d get 4 or 5 layers on and then call it a day.
Anymore layers you might find gravity gets very irritating. (unless you have a vac bag)
A layer of peel ply would be really good if you have it.
Next day, remove peelply or prep the area again like you did after day 1.
Decision time on whether to apply more layers.
If so rinse and repeat previous day.
If not, coat with a two pack epoxy primer like Intra protect, Jotun Penguard hb or similar.
Then a tie coat primer and then antifoul.
Simples.

 

[Graham376]

Peel the wet cloth off the ply/cardboard and stick it to the hull using a wideish brush and your now sticky gloved fingers.
As you’ve already wetted out the damaged area it will be easy to fully wet out without having half a litre of epoxy dripping down the brush and up your arm.
(you can make a little masking tape dam round the brush handle which will catch some)
I’d get 4 or 5 layers on and then call it a day

Surprised you don't mention compacting rollers, using brushes usually leaves the repair resin rich which isn't as strong as thin layers of resin between glass layers.

 

[Javelin]

Surprised you don't mention compacting rollers, using brushes usually leaves the repair resin rich which isn't as strong as thin layers of resin between glass layers.

Compacting rollers are very useful as long as you use the right size for the job and you are very careful not to pick up any loose strands or holding cotton, especially when working upside down.
I Thought I'd covered the resin rich issue(which I fully agree with) by only wetting out around 80%.

The techniques of applying resins, cloths etc are probably beyond the scope of this kind of thread, just ask the two aprenticies who follow me about the yard.

 

[Refueler]

A pity Strand Glass disappeared, those workshops/classes were very useful, did a couple in the 70s when I was working on grp cars.

Totally agree .... I was introduced to them by a guy who used their moulds and gear to produce Canoes and Dinghy's ..... Portsmouth ...