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Gelcoat not curing

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I removed a small fibreglass fairing piece off my sportsboat and stripped and sanded it. The previous owner had rattle-canned it in an ugly blue that had scratched badly. It wanted to strip it, sand it back and re-gelcoat it. Then sand and polish. 
I applied the gelcoat and it looked great but the only problem is that it hasn’t hardened fully. I did some research and it appears that I should have added wax to the gelcoat mix and I may have used too much catalyst as it was very difficult to measure 100:1 for a small batch.

is there any way to get this to harden or do I need to scrape it and reapply? Or can I put another coat of gelcoat over this? It’s not super soft, just slightly tacky. 

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Yes you can put gelcoat over the tacky gelcoat.

Google PVA and finishing gelcoat (I think its called). Normal gelcoat stays tacky by design, this allows multiple coats to be sprayed into a mold and have them adhere to each other.

The top coat of a gelcoat refinish needs either PVA to seal it or use a gelcoat that has wax in it. I've succesfully added liquid Johnson wax in the old days to make a 'finishing' gelcoat. i dont know what the formulation of waxes is today so I'd be careful there.

As for measuring the catalyst, any small didgital scale will do, I use a Harbor Frieght one, cheap as can be. You really need to measure the volumes correctly. But I have personally put a layer of gelcoat over a tacky coat succesfully.

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heat helps curing heat lamp heater hair dryer anything that gets hot  like an old style lamp in a box

I have used a kerosene lamp on a beach with cardboard wind blocks

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Spray it with PVA or use a cheap hair spray that is made of PVA, or if it is sticky but not soft, wax it with carnauba wax.  All you are trying to do is seal it from air so that it can fully cure[the sanding wax rises to the surface during the cure to do this].

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depends on how much to much catalyst you have used

it will compromise the strength and durability of it .. even with only a little to much

my advice .. get some acetone and remove it completely

if its just air stopping the cure then adding another thin layer of waxed and smudging it in with a toothbrush will work

how to tell

if you can scrape through it with a metal tool without to much force .. its to much catalyst and it all needs to come off .. if it just moves the surface its an air problem and can be improved

( metal tool means... spoon .. butter knife etc .. ie blunt)

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** ignore the first six replies


1.add another coat the same way you did the first ( Target 1% to 3% catalyst ) 

2. Wait

( 1and 2 should really be repeated  for a total of three or four coats. Unless you didn’t use enough catalyst, Each additional cost will kick the surface goo coat beneath ) 

3. rinse off the gooey stuff (Acetone.. paper towels... DRIPPING wet paper towels. Rinse until the acetone runs off as clear acetone) 

4. sand and polish 



If you fail, hire somebody 

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On 3/1/2020 at 3:06 PM, Gouvernail said:

** ignore the first six replies

thats a bit ott unless you have spent a decade laying up grp and another decade doing R&D on it


if the first layer touching the original boat laminate isnt cured properly nothing will get down to that in enough areas to cure it all

you can bodge it and make it look like its ok .. but it never will be


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3 hours ago, phill_nz said:

if the first layer touching the original boat laminate isnt cured properly nothing will get down to that in enough areas to cure it all

Study up on cross linking.

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On 3/1/2020 at 9:07 PM, phill_nz said:

thats a bit ott unless you have spent a decade laying up grp and another decade doing R&D on it


if the first layer touching the original boat laminate isnt cured properly nothing will get down to that in enough areas to cure it all

you can bodge it and make it look like its ok .. but it never will be


You are correct about the possibility the first layer was not sufficiently catalyzed to cure at all. 
I am assuming the writer was being honest about adding a decent amount of catalyst and I assumed the gelcoat had kicked but remained gooey on the surface. 

As for your opening sentence, I sort of had my first ten years in by the mid seventies. I did a lot of layup myself but the company kept promoting me and giving me various other positions and I did four years of college and worked at least for some time doing every “production job” in a factory that produced about 300 fiberglass boats every day. 
In other words, I have the experience you described but it is mostly irrelevant for solving Dino’s problem. 
Why??? ... The Manufacturing experience where every department had at least some people with decades of experience never exposed me to the  sort of problems encountered by beginners making repairs in a small shop or at home.

My experience gained by problem solving for frustrated boat (and other fiberglass structure) owners who have set out to “do it themselves” mostly has been provided by interacting with customers and friends since opening my own boat shop four decades ago. 
     Hardly a day goes by when I fail to receive a phone call from somebody who could use a smidgeon of understanding and advice. 
     Many of my most loyal and eager to part with money customers are folks who have previously bit off a tad more than they had cared to chew. 
     My shop has all the tools I want and all the materials I think might soon want and  I am well aware of how much more difficult even the most simple fiberglass and gelcoat work is when all those tools and materials are not present. The fact is, there is nowhere local to buy a huge percentage of the tools and materials I consider to be essential to successfully accomplishing simple repairs. 
       I spent a lot of years running my shop with neither all the right tools or all the right materials. Many times I had to do things over to achieve the quality and durability I had to create. Having “been there and done that” on virtually every mistake a guy or his new employees can make, I usually know exactly how nasty the struggling guy’s situation has become.

      My intention is to suggest the simplest way out of the jam.

    @phill_nz??? You may be right. I should have suggested the following:

@Dino before trying to add more layers, you need to be certain that first layer has cured adequately for it to serve as a first coat.

Grab some acetone and some paper towels and try to entirely wash off a small area of your not quite cured gelcoat. Do NOT use a semi wet towel. If the towels aren’t dripping rapidly they are too dry. (If it were practical to do so, spraying the acetone with a garden hose while gently wiping with a natural sponge would be the right amount of rinsing. )  Rinse until no more gelcoat comes off and the acetone running off the surface is crystal clear. 
If  all the gelcoat comes off, you will have saved days of trying to remove a god awful mess where you would have had hard gelcoat over the still gooey un-cured  gelcoat.

Sorry I  did not suggest this earlier. 
You should thank Phill!!! 

If, when rinsing your little test area, a whole lot of goo comes off but a whole lot of gelcoat does not rinse away, I am going to stay with my originally suggested game plan. MAKE CERTAIN you catalyze at 1% to 3% and start adding layers. 
as long as I am filibustering:

If you use less than 1% MEKP you will not achieve a sufficient dispersion of  the catalyst to cause consistent curing of the plastic. If you go a little over 3% you probably won’t ruin the Final plastic. 
if the gelcoat foams as you stir in the  catalyst you are absolutely pushing the high end of tolerability. If you keep stirring for thirty seconds or so and the bubbles all go away, your gelcoat is probably till OK but it may not weather as well as it could have. 
But... The adjacent old gelcoat is probably a bit old and Poitou from weathering and sometimes a little over catalyzation of the fresh gelcoat helps it to look and feel more like the adjacent old gelcoat,  Sometimes it looks like shit. 



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i actually replied to the next comment about check linking .. but got  a bit snarky so its hidden

i had about the same as you for the first 10 years .. fake marble / boats and general

then went back to school to do engineering .. then ended up working mostly in labs

the last 12 odd years were all R&D gelcoats and resins

also did tec rep if none of the salesmen or other chemists could solve the problem ( anzol / valspa australia )

i did all the comparative tests with every other gelcoat i could find / beg / buy / steal from wherever


some things were common to all badly catalyzed gels

the mechanical s always suffered .. the weathering specifically


and this particular problem i came across when my partner gelcoater sprayed a 30 foot hull without cleaning the filter for months .. eventually it got so blocked and using a flow type metering on the external mix gun which carried on at its supposed 2.5% none of the hull went off properly .. it was tacky on the outside and rubbery under that

it took me 3 days non stop to strip it back completely ..  ( he took a week off as he didnt want to clean his mistake .. obviously cheering me up no end )

as i was taking the last bits off it had firmed up a lot but nowhere near what it should have been


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@phill_nz,,, external mix with a catalyst tank??

did you have a Binks spray gun with a tiny injector that squirted catalyst next to the spray cap into the exiting fan of gelcoat ??

i bought a Binks unit as described from an old friend and I am afraid to use it.

my experience with similar equipment had the catalyst mixed into the atomization situation supply .  I know how to set up, trouble shoot, and maintain that system but the tiny injector nozzle seems like a “didn’t get it right” just waiting to fuck up life. 

if you have used the Binks system I would live to pick your brain 

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sorry for the hijack but gov has messages turned off


hi .. glad to meet an old styrene sniffer thats not dead

2 immediate questions

was it a binks bullet

did i answer your question on it

as an after thought

do you want any other info i might know about it



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I must have misled you.

The spray system I bought from my old buddy is at heady 35 years old. He hasn’t used it in 30 years and he totally cleaned it before storing it. 
 The 1960s version of the system used a Binks model 18 gun. One hose delivered gelcoat to a fluid nozzle that put out a stream of gelcoat through about a one or two mm hole 

the other hose delivered air through an air cap with about ten tiny holes. The gun had needed to adjust the fluid rate and the air jumpers could be adjusted to change the pattern and size of the spraying fan of material. 

the system included a canister through which the compressed air passed on its way to the spray gun. The canister was filled with a supply of MEKP. The canister had various adjustments that allowed the operator to control the amount of MEKP was added to the air.


Quickie summary: The gelcoat coming out of the Gun was atomized by air mixed with MEKP. 
catalyzation rate was regulated by air pressure, the Canister’s MEKP delivery rate, and the gelcoat flow rate. 
 As long as the air and MEKO mixture was breaking the gelcoat into relatively fine particles, the resulting gelcoat cure was near perfect 

stepping ahead to my old friend’s new “modern” 1985 gun.

Why change?
People didn’t like the idea of blowing a mixture of MEKP and air through a spray gun. Also, whenever the so ray gun was shut off, the MEKP would settle in the hose. (The hose between the canister and the spray gun had to be about fifty feet long to reach all around a 20 foot boat ) The process for spraying a mold included spraying air until the settled MEKP had blown through and the gun was producing a consistent mist of MEKP and air.

That Pre-spraying with the gun aimed at the exhaust fan set up a lot of exhaust systems for wonderfully nasty fires. 

so... Binks cane up with a system with a third hose and a tiny injector tip which was mounted on the side of the model 18 gun. 
How it is supposed to work 

gelcoat comes out through the fluid nozzle 

air comes through the air jets and atomizes the gelcoat 

the tiny nozzle squirts MEKP into the fan of gelcoat and enough mixing occurs to create well mixed consistently catalyzed gelcoat.

I am not using the system until somebody who has EFFECTIVELY used it has a long chat with me about his success.


Finally... the MEKP canister seems to be the exact same thing  as was used for the air/MEK mixing system. 
my guess is it has different internal parts which I could buy from Binks 

currently I do not need to build anything as large as a boat so I do not need a decent large volume gelcoat spraying system . I do not yet need to figure out how the old friend’s system works and I don’t need to modify it to be like what I used to build 200 boats every week in the early seventies 

and... unless I start doing some sort of bass production again I have no reason to invest in a modern airless system 


I just want to run into somebody who has used the side drop injector system Binks sold my friend sometime around 1983 when somebody at Binks must have thought it was fantastic 




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So I just learned a lot more about gelcoat than I ever planned on, but then I already knew that some things are just better to pay a Gouv to do. It's like texturing drywall but harder.  Good info though and am glad to know it.  I can patch but I can't match.

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i selected hide on my first long reply to your question

as i did some research and found binks has changed the way they catalyze and the models of their guns .. a lot

the reply was specific to the binks bullet .. a gun i first came across in roughly 82 and last in 99

in short

it was by far the best sprayer for gelcoat i have ever used / come across

it was also by far the worst catalyzing gun ever invented

because they used a wide angle between resin and catalyst sprays they didnt meet till about 150 -200mm from the gun .. whereas every other external mix gun i have seen / used the range was 100mm max ( closer to 50mm )

that created a lot of problems they never solved

the 2 biggest i found were .. an orange peel effect on the gelcoat surface .. and .. lines of pin holes you could see if you shone a light from the back of the laminate

they were responsible for a few of the problems i was called out to figure out as a tec rep

my advice to the users was in short dump them and buy something that worked


because of the lack of info on the web for pre 2010 guns and the amount of guns binks went through you will have to take a pic of what you have and i will see if its close to what i remember .. or is likely to have the same problems

all the catalyst lines on external mix guns i have used required a short pull on the gun to set the catalyst flowing correctly ( judged by eye ) then a further pull to start the resin

as gelcoaters go they are ok .. not as good as an internal mix gun but acceptable .. and use less solvent wash


the new binks external guns look really weird and i would have to look into them and the exploded diagrams to make any comment on them at all


perhaps start a new thread on anarchy fix called binks and external mix guns .. or we can set a legacy thread and call it gelcoat guns .. problems and fixes


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But Hijacking threads  is such fun 


I suspect I should simply get ahold of Binks and see is anybody is old enough to know about the toy I have

Edited by Gouvernail
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very true

but that one might be worth keeping ( legacy info before we all kick off )

pretty sure from the years you mention it is a binks bullet .. they sold them hard even after the problems started getting to be known

they were so proud of them at first .. game changer .. then it was

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