How Deep Is The Gel Coat

Little Dorrit

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I am rubbing my very old and tired GRP Hull down with a light 400 grit sandpaper before farecla 3 then 6 and then polish.

A test seemed to show the results are foing to be good..however I keep getting strange looks and warnings not to go through the gelcoat.

How can I tell how how thick the gelcoat is & how will I know when I have overdone it?
 

Cymrogwyllt

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I am rubbing my very old and tired GRP Hull down with a light 400 grit sandpaper before farecla 3 then 6 and then polish.

A test seemed to show the results are foing to be good..however I keep getting strange looks and warnings not to go through the gelcoat.

How can I tell how how thick the gelcoat is & how will I know when I have overdone it?

I rammed a sea kayak (vyneck) in to the steps of Wye rowing club during a break out at the end of the wye marathon. It hit hard. The loss was a chip of gell a couple of square inches. Never bothered to fix it.

If the glass is not exposed/ broken then a cosmetic glass over will sort it
 

Avocet

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Typically, about 0.3mm - 0.5mm thick, but can vary considerably. On a flat panel, or one with a gentle curve, like the side of the hull, the thickness will probably be quite uniform. On sharp external corners (like the outer edges of a rubbing strake), it is likely to be much thicker - up to 3mm quite easily. On an internal tight corner, often less.

In the course of stripping Avocet's off I've been going down about 0.3mm and there are patches where I can still see the remains of some gelcoat at the bottom of the cut.

You'll know when you're going through because you'll start to see the glass fibres becoming exposed. That's bad, because water will tend to "wick" up the bundles of fibres. It's easy to see if the resin was un-pigmented, (it will be brown-ish), but if it was pigmented, it could be the same (or a similar) colour to the gelcoat. Hand polishing is VERY unlikely to go through unless you get a patch of unusually thin gelcoat. Even then, I'd be surprised!
 

alan17

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Gel coat

I am rubbing my very old and tired GRP Hull down with a light 400 grit sandpaper before farecla 3 then 6 and then polish.

A test seemed to show the results are foing to be good..however I keep getting strange looks and warnings not to go through the gelcoat.

How can I tell how how thick the gelcoat is & how will I know when I have overdone it?

A section of the real question is "How canI tell how thick......."

I know of no way of knowing until it is too late! If you must use wet and dry use very fine. Probably 1200 grit andas little as you can.
 

Marine Reflections

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I am rubbing my very old and tired GRP Hull down with a light 400 grit sandpaper before farecla 3 then 6 and then polish.

A test seemed to show the results are foing to be good..however I keep getting strange looks and warnings not to go through the gelcoat.

How can I tell how how thick the gelcoat is & how will I know when I have overdone it?

Agree with all the advice above.

400 is a little harsh, but may well be what is required to reveal fresh gel, I would go with taking twice as long and twice as careful and use an 800 grade followed with 1200 / 1500 wet. Go with the least aggressive first, you can always work that section a little more, but if you go too fast first and go through, then there is a lot more work for you that you had not planned on.
Not only that, even if you don't go through this time, you may next time or the next owner will.

If your gel is older than most there is every chance it has thicker gel than modern moulds, so a bit of good news.

Also a huge ask of any product to bring back from 400, it will leave your new fresh 'thinned' gel looking shiny and uniformed, but very close up will be rough and open to attack from oxygen once again, certainly if you then trap air in the pores with a polish. It will continue to oxidise and break down again.

With regards to a gauge for thickness, you really can't tell until it is too late.

With a cars paintwork for example you can buy a paint thickness gauge to determine an overall thickness. This works by reading the distance from the metal and includes primers etc and gives a reading in microns.
You would then get to see any areas of repair or rubbing down including high points.

With gel coat it is possible but very expensive. Sonic machines are available but would not be in the least cost effective at many thousands of pounds.

You can see any thin areas of gel when new (or by stripping your interior) by using bright lights to shine through the fibreglass, if an area is thin then more light will come through.

If you have tried it on sections and the results are good, then keep doing what your doing, just slower or softer.
:)
 
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late-night-lochin

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Don't try to achieve perfection

I am rubbing my very old and tired GRP Hull down with a light 400 grit sandpaper before farecla 3 then 6 and then polish.

A test seemed to show the results are foing to be good..however I keep getting strange looks and warnings not to go through the gelcoat.

How can I tell how how thick the gelcoat is & how will I know when I have overdone it?

You've already had some good answers but there's one thing I like to stress. Do not try to achieve perfection because some marks will be so deep you will almost certianly go through the gel coat if you try and remove them completely.

in addition to surface scratches, your old gel coat will probably be oxidised, faded and maybe stained. 400 grit is a bit harsh but you probably do need it but only try to get rid of the worst of the imperfections then work through finer grades before polishing. These finer grades will still be removing more gelcoat so by the time you've finished with the 1200, it will be even better than it was after the 400. You can still go through the gel coat when you're working through the finer grades.

When you do the job, you will be studying the gel coat from about 12" away and will see every little imperfection and that's when you're in danger of sanding too much. Walk away and look then don't try to get rid of imperfections you can't see from 6' away

Why do I say this with such confidence? because I've done it then had to re-gel coat the area just because I hadn't learned when to be sensible and stop!

Best of luck
 

Marine Reflections

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You've already had some good answers but there's one thing I like to stress. Do not try to achieve perfection because some marks will be so deep you will almost certianly go through the gel coat if you try and remove them completely.

in addition to surface scratches, your old gel coat will probably be oxidised, faded and maybe stained. 400 grit is a bit harsh but you probably do need it but only try to get rid of the worst of the imperfections then work through finer grades before polishing. These finer grades will still be removing more gelcoat so by the time you've finished with the 1200, it will be even better than it was after the 400. You can still go through the gel coat when you're working through the finer grades.

When you do the job, you will be studying the gel coat from about 12" away and will see every little imperfection and that's when you're in danger of sanding too much. Walk away and look then don't try to get rid of imperfections you can't see from 6' away

Why do I say this with such confidence? because I've done it then had to re-gel coat the area just because I hadn't learned when to be sensible and stop!

Best of luck

The deeper scratches need to be filled with fresh gel then sanded along with the rest of the surface.

Do try and aim for perfection as the end surface profile will be providing the main protection. A polished 'still scratched' surface will not provide the same or look anywhere near as good. A torch and some sunnies will help.... a lot!

100% correct you should not try and go for 'base' by wet sanding alone if the scratches are too deep, of course you will go through.

Try the whole process on a section. By this I mean finish the section, have it fully polished with the final stage as correction marks may not be possible to see until fully polished up.

6' rule? I'd be out of a job :eek:

My mum used to refuse to wear her glasses when doing the housework, she would say it would take ten times as long. :)
 

Marine Reflections

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Just out of interest what is the micon difference beteen 1200 and 600 grit - is it 600 microns?

1200 grit has 1200 particles per square inch, 600 grit has 600 and so on.

A micron is a unit of measure, there are 1000 microns in a mm.

The better the paper the more guarantee you have that the particles are of correct size throughout the entire sheet.
 
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Marine Reflections

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My god you all must have very thin gelcoats? Having taken a closer look today I noticed that a 2mm "dent" has only just gone through to the fiberglass!! Perhaps I am pretty safe??

No you are not safe... A 2mm dent / scrape in gel will need to be filled rather than abrade down.

I imagine your gel is thicker than most, but do remember that the thickness will be thicker/thiner all over to some degree, don't take that reading as being all over.

It is the reason things are done slower, safer with more effort and less grade, as in 1200, 1500, 2000, 3000, compound, finer compound/polish, de-grease, sealant (gel sealer) then wax.

That is of course generally speaking, a good order in what to do to provide a fantastic gel surface. You may try just compound on it's own and decide from there what imperfections that 'pass' has removed, repeat a few times if needed and hey presto, all the correction has been done with minimal abrasion.

Without trying for the least aggressive first you won't get a safe feel for the work required. The main aim is to remove only exactly what is dead, or uneven in surface profile and no more.
 
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Avocet

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My god you all must have very thin gelcoats? Having taken a closer look today I noticed that a 2mm "dent" has only just gone through to the fiberglass!! Perhaps I am pretty safe??

Whereabouts was your "dent"? In some areas (particularly tight external corners, it's very hard to persuade the glass fibres to follow the curve of the mould, so you end up with a gel-rich patch. It's (as you've seen) far more brittle than a thinner gelcoat with a decent glass laminate immediately behind it. Some early fibreglass boats were, I believe "double-gelled" so that they had twice as much gelcoat on them as could be applied in one coat, but its rare to find one with a uniform gelcoat more than about 1mm thick.
 

Marine Reflections

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The difference in thickness can be so dramatic within inches from each other.

Take a look at the thickness difference on either side of the chip.

Agree 100% with Acocet, some areas just get a build up of gel, others get air trapped giving the impression of the opposite.

You just cant be 100%, certainly with older gel. It is rare to find one that has not had problems, repairs, over-correction and the odd neglected season here and there.

You get a feel for it over time and can tell if you are 'on a rizzla' paper gel or a Jacobs club gel.

Remember of course it's purpose is to protect the fibreglass really and that's it. How smooth, reflective and interactive with the elements you make it, that is what it is there for, how thick does it need to be...


tanameraJune2010surfacecorrection036.jpg
 

Marine Reflections

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A bit easier to see in a different light, a pic of thin gel below, note the contours of the surface.
Human error was at fault, a previous repair had been done on an area next to the 'thin'.
The repairer performed a repair, then polished to blend, however was a little over eager on the high point of the contoured surface. Result being a thinned area of gel.
The whole panel would now be rather suspicious.

P1070263.jpg
 
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