Gelcoat pox (after blasting) - how to fill and fair before Penguard?

[dankilb]

Suggesting on other threads that people slop Penguard HB anywhere/everywhere on their boats(!) has got me thinking about my own impending showdown with this... the gelcoat as it looks after blasting back in Oct:

The plan is for Jotun's Penguard HB (already purchased) up to 5 coats and then Jotamastic 87 for a further 3 coats. It needs to be moisture resistant as possible because, in the process of 'energising' the hull with Kevlar, Jeanneau left lots of small voids in the inner laminate that suck moisture up like a sponge. Luckily I'm guessing the whole lot is pretty porous (or we caught it just in time) because the hull is now dry leaving behind only these small gelcoat blisters after blasting.

Question is - what is the easiest way to fill and fair areas (small blisters; light 'pox'; think gelcoat, blasted through) prior to applying the Penguard?

a) Sand whole areas back as much as possible and aim to build it up with the high-build ('HB') primer and some minimal filling/fairing with thickened epoxy where required?
b) Leave the 'pox' as is and fill with a dab of thickened epoxy? Then sand back until fair?
c) Grind out each one (not too bad with a die grinder) and fill/fair?

As you can see from the above options, I'm not considering 'proper' filling and fairing (laying on a whole coat and sanding/longboarding it off). I just don't have the time or energy. A 'good enough' version will have to do. I'm confident I can sand relatively fair with my large random orbital with abranet discs on a soft pad.

All thoughts or similar experiences gratefully received!

 

[Concerto]

I would use Teroson UP620 Gel Coat Filler to fill the holes. This is a 2 part polyester based filler, previously called Plastic Padding, that is quick to mix and apply, fast to set, and quick to sand smooth.
Teroson UP620 Gelcoat Filler 241g

 

[Elessar]

Get a solvent free epoxy. Such as international gel coat plus. NOT 200. That’s solvented.

Thicken the epoxy and fill and smooth off the worst of the holes with a spatula.

Within 24 hours paint on a thick coat of epoxy and smooth it off with a wide wiper designed for getting water off glass or cars.

Do it again within 24 hours.

Roll on a coat of solvented epoxy within 24 hours.

Waterproof. Smooth enough. And with an infinite overcoating window without sanding.

Timings and the solvented/non solvented bits of the instructions are very specific.

 

[dankilb]

Thanks @Concerto, hadn't occurred to me to use a polyester filler. The ease of application/sanding is definitely tempting. Unfortunately the cost, at least of the marine varieties, seems prohibitive to me. For instance, a quick calculation suggests my usual methods of thickening West's epoxy work out at about £8 for the same quantity. If I used a cheaper resin, e.g. EL2, that would halve!

Get a solvent free epoxy. Such as international gel coat plus. NOT 200. That’s solvented.

Thicken the epoxy and fill and smooth off the worst of the holes with a spatula.

Now that had occurred to me - in other words, just thicken the epoxy-based paint. I wasn't sure whether anyone had tried this? I have already acquired the paint I plan to use (Jotun Penguard). That's a non-solvented, amine-based epoxy with a 16 hour to 3 month self-overcoating interval. It's already pretty thick, so I reckon a small volume of fairing filler powder (or my own mix) would thicken it enough to fill the small voids in the gelcoat.

I'm not sure I could do a proper 'hot coat' method (recoating within hours or a day or so) just because of the area involved, my own availability, N Wales weather, etc.!

I love the suggestion of a wide squeegee/wiper - or even an old wiper blade perhaps? - to fair off larger areas. I really just need to get some coating into the lighter 'pox'. They're mercifully very shallow, not into the laminate. But my experience of coating the keel with similar epoxy paint before lockdown suggests it won't quite fill small imperfections itself off the brush/roller - it leaves a 'pumice-like' surface (that then needs fairing!).

 

[Bobc]

Listen to Elessar. He knows what he's talking about.

 

[BabaYaga]

I used a counter sink drill bit to open up the blisters/cavities a little, then Plastic Padding marine epoxy to fill them.

 

[dankilb]

I used a counter sink drill bit to open up the blisters/cavities a little, then Plastic Padding marine epoxy to fill them.

Good tip. I had thought of 'dishing' them open slightly with a domed bit on my die grinder (or just a cordless drill, it's hardly going to offer much resistance!).

I'm very disinclined towards any more 'involved' grinding/bevelling as that could set me down a whole path of unnecessary labour!

I was also tempted by either a) not sanding/machining at all and just squidging my chosen fairing compound in to the pox marks, or b) sanding the worst areas to actually thin the gelcoat down to almost-fair (on the basis that, by the time it's in this sort of state, it isn't really 'doing' anything anyway...).

Thankfully large areas of the gelcoat are sound. It gets worse approaching the topsides. Maybe the gelcoat flowed down the mould - getting thicker towards the keel?! ?

 

[BabaYaga]

It is many years since I did this work, but IIRC quite a few of the smaller looking holes actually had larger cavities under them (I guess this depends a lot on the aggressiveness of the blasting...) Therefore the drill bit.
The PP epoxy filler is quite 'thick', I don't remember there being much need for fairing after filling the holes.

 

[PCUK]

You must use the epoxy primer first. Working it well into the holes with a brush. Time consuming but if you fill first you will leave unfilled voids under the filler as air will prevent it fully filling the holes.

 

[dankilb]

So I think I've got a method in mind, based on the above:
1) Open up any larger pox
2) Coat of unthickened epoxy primer in those areas needing filling (working into the pox as @PCUK suggests)
3) Fill pox with thickened epoxy (still minded to use thickened Penguard rather than thickened laminating resin)
4) Fair the 'pumice'-like/uneven areas with a squeegee (or even a wiper) using lightly-thickened epoxy, as per @Elessar's method (but with the Jotun, because that's what I've got!)
5) Light machine sand, as required to flatten everything out
6) Start building up the coats of epoxy primer

Overall I'm happy with the idea of this as it doesn't feature grinding, torture boards, or anything else that risks unnecessarily eating into the time/labour.

 

[BabaYaga]

You must use the epoxy primer first. Working it well into the holes with a brush. Time consuming but if you fill first you will leave unfilled voids under the filler as air will prevent it fully filling the holes.

I might well be wrong, but this seems a bit unnecessary to me. From memory, Plastic Padding epoxy filler doesn't require a primer. If you have a void, conical in shape (from the counter sink bit), I do not quite see how/why air should get trapped under the filler. Even if so, microscopic air pockets in the laminate or interface gelcoat/laminate might not be a problem anyway?

 

[dankilb]

I might well be wrong, but this seems a bit unnecessary to me. From memory, Plastic Padding epoxy filler doesn't require a primer. If you have a void, conical in shape (from the counter sink bit), I do not quite see how/why air should get trapped under the filler. Even if so, microscopic air pockets in the laminate or interface gelcoat/laminate might not be a problem anyway?

Isn’t the idea basically that epoxy adheres best to itself? So getting a thin layer on first may help any subsequent filler to adhere even better, as well as being more likely to flow into any small voids.

But I’d also hope that it’d make little practical difference, as the stuff adheres well anyway, plus it’ll be encased under many coats of epoxy primer.

Weird things can happen to coatings below the waterline, in my limited experience, so best not give it any excuses to misbehave, I guess?

 

[dankilb]

Also interested to hear from anyone who’s done this with Penguard HB specifically. I know it has many proponents!

I’m still unsure about a few things, like:

Sanding between coats (I assume it isn’t really necessary, but if I need to in any areas, does 180 grit sound about right?)

Using a guide coat - which was recommended to me by SML - but I’m not sure I really need (it’s not a racing hull!)? Then again, slapping on a coat of white over a contrasting colour and sanding to reveal any low spots or pinholes sounds easy enough to do...

And has anyone tried an epoxy mastic on top (Jotamastic)? It has higher water resistance, through an aluminium additive like West 422 barrier.

 

[Elessar]

Isn’t the idea basically that epoxy adheres best to itself? So getting a thin layer on first may help any subsequent filler to adhere even better, as well as being more likely to flow into any small voids.

But I’d also hope that it’d make little practical difference, as the stuff adheres well anyway, plus it’ll be encased under many coats of epoxy primer.

Weird things can happen to coatings below the waterline, in my limited experience, so best not give it any excuses to misbehave, I guess?

So many errors in this. For example solvent free epoxy won’t stick to itself at all after 24 hours. You say “epoxy” like it’s one product and your post is just all wrong.
What is “epoxy primer”. A primer for what? “Epoxy sticks to itself”, well it might, in might not and it sticks to other stuff too.

 

[Elessar]

Also interested to hear from anyone who’s done this with Penguard HB specifically. I know it has many proponents!

I’m still unsure about a few things, like:

Sanding between coats (I assume it isn’t really necessary, but if I need to in any areas, does 180 grit sound about right?)

Using a guide coat - which was recommended to me by SML - but I’m not sure I really need (it’s not a racing hull!)? Then again, slapping on a coat of white over a contrasting colour and sanding to reveal any low spots or pinholes sounds easy enough to do...

And has anyone tried an epoxy mastic on top (Jotamastic)? It has higher water resistance, through an aluminium additive like West 422 barrier.

Sanding between coats may be essential, unnecessary or undesirable.
Depends on the type of epoxy and the time between coats. I’ve no idea what type of epoxy Penguard HB is. With epoxy don’t assume always RTFM they vary hugely.
180 grit a bit smooth as a key. Between 80-120 is what I use.

 

[dankilb]

So many errors in this. For example solvent free epoxy won’t stick to itself at all after 24 hours. You say “epoxy” like it’s one product and your post is just all wrong.
What is “epoxy primer”. A primer for what? “Epoxy sticks to itself”, well it might, in might not and it sticks to other stuff too.

Okay, I see where I've gone wrong here - Penguard HB almost certainly is solvented. (so apologies to anyone confused by my statement above!)

I say almost certainly because I've of course studied the TDS and Application Guides very closely, but it doesn't state anywhere. But I've also used it before and it stinks (requiring respirator) and can be thinned with a solvent thinner - so I think that answers it. It also has a minimum overcoating interval which, to my limited knowledge, also suggests solvented.

I now also looked properly at International Gelshield Plus and can see it behaves like a laminating resin, in that it requires 'hot coating' (before amine blush etc.).

Problems are that a) I have the Penguard already and b) I don't think I can use a 'hot coat' method, at least not over the whole hull, as I'll likely be on my own (for 42'), plus I can get called away from time to time and the weather can also be unpredictable in this part of North Wales.

So I'm suggesting doing all of this with a solvented epoxy primer, is what I'm saying. Really I shouldn't say 'epoxy' but 'the epoxy' (i.e. the one I've already bought!). My plan is also to try using the same primer, slightly thickened, to fair the pox.

I have various laminating resins (indeed, even West's is cheaper than Gelshield Plus) and - while it might not sound like it - I do have plenty of experience using both those types of epoxy (resin) and this particular paint system. It's just the state of the substrate that's foxing me.

 

[Tradewinds]

Problems are that a) I have the Penguard already and b) I don't think I can use a 'hot coat' method, at least not over the whole hull, as I'll likely be on my own (for 42'), plus I can get called away from time to time and the weather can also be unpredictable in this part of North Wales.

Why not just do half the hull ie one side at a time. That way you just have to abrade a small joining strip fore and aft when you do the other side. That's what I did on my Nic39 many years ago. Saved a lot of worry & rushing about.

 

[dankilb]

Why not just do half the hull ie one side at a time. That way you just have to abrade a small joining strip fore and aft when you do the other side. That's what I did on my Nic39 many years ago. Saved a lot of worry & rushing about.

I had thought about doing it in sections. However, I remain quite reluctant to use a 'proper' (non-solvent) epoxy in the process at all, if I can possibly avoid it.

With the Jotun paint system, you're supposed to get very high moisture resistance without having to worry about wet-on-tacky application or overcoating intervals at all. Essentially, once I've got the hull fair, it's just conventional roller painting with as-good-as-unlimited intervals in between.

Given that the previous gelcoat was presumably offering very little moisture barrier, I'm hoping that my proposed system will be considerably 'better than nothing'.

If I had the time and inclination, I would 'hot coat' with a laminating resin (plus West 422 barrier additive), but I know what's realistically involved in this and I'd need an extra pair of hands, just to help with mixing, to get the required coats on in time, even over a section of the hull.

I'm sure the International Gelshield Pro is a great product, but based on their application instructions and estimated coverage on my hull we'd need up to 20L so over £700 ?! That cost is equivalent to enough Jotun paint to do the whole job several times over. Plus I've already got the paint, anyway!

 

[Tradewinds]

Yes, we hot coated with West with the 422 additive. There were three of us. One mixing, one rollering and one tipping off. As you said, hard work on your own. Will be interested to know how the Jotun Penguard works out for you as I have an upcoming project I could use it on.

 

[Elessar]

I had thought about doing it in sections. However, I remain quite reluctant to use a 'proper' (non-solvent) epoxy in the process at all, if I can possibly avoid it.

With the Jotun paint system, you're supposed to get very high moisture resistance without having to worry about wet-on-tacky application or overcoating intervals at all. Essentially, once I've got the hull fair, it's just conventional roller painting with as-good-as-unlimited intervals in between.

Given that the previous gelcoat was presumably offering very little moisture barrier, I'm hoping that my proposed system will be considerably 'better than nothing'.

If I had the time and inclination, I would 'hot coat' with a laminating resin (plus West 422 barrier additive), but I know what's realistically involved in this and I'd need an extra pair of hands, just to help with mixing, to get the required coats on in time, even over a section of the hull.

I'm sure the International Gelshield Pro is a great product, but based on their application instructions and estimated coverage on my hull we'd need up to 20L so over £700 ?! That cost is equivalent to enough Jotun paint to do the whole job several times over. Plus I've already got the paint, anyway!

The advantage of solvent free is that wet film thickness = dry film thickness.

If you blade smooth it over the dimpled surface there is zero shrinkage. Then put a coat of "the epoxy" over that during the overcoating window and you have you surface that doesn't need sanding with a minimum overcoating window (and no max). Quickest way to a good enough for underwater finish by far.

From memory wetfilm thickness of gelshield 200 is about 90 microns and dry film about 40 (don't quote) but the point is solvented epoxy shrinks a lot.