Thin gelcoat

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beachandboat

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Hello,
I have just stripped my GRP hull back to its original gelcoat. I do not intend to apply any more anti-foul as the boat will be trailered.
The plan is to wet and dry, cutting compound and then apply several coats of polish.
The problem being is that in several places the gelcoat has been sanded through to show the darker resin and matting.
Should I build up some gelcoat filler over these areas and sand to suit, what would be your recommendations?
Thanks in advance
 
I think the problem you will have is to colour match the new gelcoat to the surrounding colour. This is IMHO a very skilled job, particularly if the repair is in a prominant position.
If you struggle to make a good colour match and then get a specalist to help you, you have made the job harder for him as you have thinned the gelcoat before he starts.
You should match the gelcoat colour by cleaning the surrounding area and matching to the cleaned area.
I don't wish to be a misery, but this is a job to be wary of starting unless you have some experience of this sort of work.
 
Thank you Alan.
I should of mentioned that the gel-coat is white. There is no edge, the places have just been ground down a little too much.
 
I'm afraid its one of the most time consuming job to do, that is why it is expensive, we would normally spray a thick gel and then hand block back to smooth.
Why dont you consider a few coats of epoxy, such as Blakes SFE, this is much harder than poly .
 
Thanks for the advice so far, its much appreciated. How thick is the gelcoat applied in the mould at point of build? is it just microns thick like a paint or millimetere's?
 
If you have worn through the gelcoat below the waterline I would paint it with Blakes epoxy primer undercoat. Two coats will be enough. It is expensive but provides a really tough surface and allows future antifouling if required.

I have done my last two boats (trailer sailers) with this sytem and it provides a really hard wearing and scratch resistant surface where the trailer rollers abrade the hull. It is easy to touch up as well.

The only difficult thing is getting the waterline marked in a straight line!
 
Steady hand a must! /forums/images/graemlins/smile.gif

I would really prefer not to paint the hull, like I say - 98% of it is perfect, there are just 3 or 4 small places, could I not over fill with a gelcoat and block it smooth to suit. Then wax the hull in its gelcoat.
Thanks
 
It depends on the laminator but gel could be from as little as 1mm if sprayed verylight, as on a racer,to 3mm on a workboat
It is a very skilled job to get it back smooth, so allthough you don't want to paint your boat , SFE is harder and a dam site easier. It is also waterproof wich gelcoat is not.
 
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It depends on the laminator but gel could be from as little as 1mm if sprayed verylight, as on a racer,to 3mm on a workboat
It is a very skilled job to get it back smooth, so allthough you don't want to paint your boat , SFE is harder and a dam site easier. It is also waterproof wich gelcoat is not.

[/ QUOTE ]
Aah! thanks boatbuilder, I thought that the gel was to waterproof the resin & fibreglass. So either treatment (SFE or re-gel) is for protection & cosmetics? I was concerned about osmosis.
 
[ QUOTE ]
thought that the gel was to waterproof the resin & fibreglass

[/ QUOTE ] It is but it's not as good as an epoxy coating. Before epoxy coatings appeared on the scene 2 pack polyurethanes were used.
Antifouling does nothing in the way of water proofing it just stops fouling if you are lucky.
If you boat is not afloat 24/7 then really a cosmetic coating is all you need.
 
For as much as I agree with Boatbuilder the problem is that a uniform thickness of Gel coat over an entire hull cannot be guaranteed due to the viscosity of the product, although modern production techniques have helped
Epoxy Paints are not a one coat wonder. they can be more waterproof (but not totally waterproof) than gelcoat but you are looking at applying at least 4/5 coats to achieve a barrier before applying Antifouling tiecoat and antifouling
 
Thanks everyone, all help gratefully received.
so as my boat is now with some thin patches of gelcoat it would not be at a heightend risk of osmosis attack unless it was moored rather than trailered?
For best cosmetic results coat in Epoxy?
Epoxy is a finishing coating, it does not have to be overcoated with an antifoul?
Thanks again
 
You will find that there are many shades of white. For example to colour match a Bayliner often a tiny bit of red and black pigment is added to the white to get the correct shade. I am not a specalist in this area but I have used a specalist for several jobs and he explained this to me.
As the Boatbuilder explained gelcoat is not 100% waterproof, it is however much more waterproof than the polyester resin that it covers. Epoxy treatments are very close to 100% waterproof and are easy to apply by roller ( if thats the way you want to go) they are a recognised system, ie it's not regarded as a bodge.
Once you have used the epoxy system ( eg Gelshield or simular) you need to still antifoul her if you are leaving her in the water.
 
gelcoat often is polyester resin. As I understand it, it is less waterproof than the laminating resin which is why the underwater bits of some boats are clear rather than pigmented.

If there are just one or two places where the gel is thin it might be worth having a go at painting with gel and rubbing back. The background you have given suggests that the boat is a smaller older one so it may well not be worth while doing a full re-paint. In any case, provided you have fully flatted the attempt to re-gel, you havent really lost anything have you? You can still epoxy or whatever afterwards if you're not happy with the gel.

For the upper hull surfaces I would have thought you would use a two pack poly rather than an epoxy anyway.
 
Clear gel is used to check that there are no faults in the construction of a hull. most commercial hulls are clear underwater in order that the survay can be done. colour gel merely has a pigment added.
By the same rule never buy a boat that has been laminated using pigmented resin, you cant see any faults.
 
Gel coat IS to protect the laminate and when applied as per the manufacturers instructions does a very good job.
The problems that arise with older boats is that, as the procedure of converting drums of Gel, resin and mat into a boat hull should be virtually carried out under laboratory conditions, this is not how the majority of our aging craft were built, more like a damp cold shed beside the water with a limited knowledge workforce, consequently thin gel, laminate contamination, damp mat, dodgy catalyst mixes etc etc were common practice of that era and "OSMOSIS" was born.
in an attempt to stop the ravishes of boat "pox" or osmosis or hydrolosis the paint manufacturers experimented with variants based on polyurethanes These poly paints gave a similar lustre and when applied to GRP gave them that good as new look, and were at the forefront of technology at that time , however when applied to underwater surfaces eventually miriads of tiny blisters would appear
So it was back to the drawing board and 2 pack Epoxy paints were trialled with resounding success, providing film thicknesses were adhered to, skimp on the number of coats and you might as well have not bothered as moisture vapours will even penetrate Epoxies especially if you have the right conditions on the hull side of the coating ie a damp hull.
So do not use two pack poly's U/W
should you not wish to spend small fortunes on Epoxies the next best product would be an aluminium based U/W anticorrosive such as vinyl tar or similar, Primocon I believe is based on this type of product but like most coatings has to be applied at the correct film thickness but you don't need an antifouling tie coat providing overcoating times are adhered to
 
This is turning out to be quite an education, thanks all!
Yes the boat is a 10 year old 18 foot fishing boat.
To clarify - the intention of the work was not to re-gel the complete hull, I have just stripped years of bodged-on antifoul to reveal a good bottom other than a couple of places where the gelcoat is thin enough to see the darker resin and matting below.
Are isolated gel repairs to these areas being discourage purely because of the difficulty in colour matching? or is this an impratical method for another reason, I appreciate it will be time cosuming.
Thanks again
 
Hi Koden and welcome to the forum.

IMHO you have received a lot of advice so far that is partially conflicting and only partially correct.

I believe you are going to keep your boat on a trailer most of the time and afloat for relatively brief periods which will affect the best way forwards for you. You perhaps should also consider future owners who may want to keep the boat on a mooring.

Gel coats are difficult to colour match. It's amazing how many whites there are and even more amazing how different they look. Gel coat provides a good barrier coat against moisture ingress but not as good as epoxy.
I think your simplest course of action will be to apply a solvent free epoxy to build a good moisture barrier and prevent osmosis setting in. It is essential that at least the first coat is solvent free epoxy as the laminate will absorb solvents like a sponge and blistering will almost certainly result. The main difficulty with this will be getting temperature and humidity right unless you can wait until the summer. Once the first coat is on you can continue with a solvent based epoxy to build enough film thickness. I believe you need about 300 microns for a good barrier coat.
Various suggestions have been to use other primers (such as Primocon) or polyurethanes. These all contain solvents and should not be used as the first coat at least. Once you have started with epoxy you may as well continue - the end result will be far better.
The downside of epoxies is their poor UV resistance. You will need to coat them with a pigmented UV barrier coat. If the boat is to be kept on a trailer most of the time and only in the water for brief periods (a couple of days maximum) you could use a 2 pack polyurethane to give a good cosmetic finish. It will, however, be easily damaged and as someone has said it is prone to blistering after a short period of immersion.
Better, in my view, is an antifouling even though you don't need the biocidal properties. It will be matt so not show marks so easily and it will be easy to touch up if you want it to look good. Hard antifouling will be better than eroding types.
Despite sounding a lot of work it is all actually pretty straightforward providing you prepare the surface well and follow temperature, overcoating and film thickness instructions
 
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