Gelcoat dilemma

[chieftaing]

1987 bayliner capri restoration; 3 coats of gelcoat, let each one cure a few days, third coat had wax - but the deck has lots of sticky areas that seem like they haven't cured. Grrrrrr.

First coat done using HVLP gun, what a hassle; 2nd and 3rd coats done with roller and tipping brush.

Fresh gelcoat from us composites. All coats thinned @ 10% using styrene, used us composites recommendations on hardener, mixed 2 minutes by hand, generally 1 pint @ a time. Last coat had wax additive following qantities on label. The remnants in the mixing pot were hard and the 3rd coat seemed non sticky.

Questions:
Can the third coat with wax be applied with roller?
How do i get this back on track? Sand top coat even if sticky? Acetone? Laundry detergent to remove wax?
Switch to paint? How to prep sticky gelcoat for paint?

I'm so close on this - help?
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[Scott Danforth]

Try spraying PVA over the gel. That should help it cure

 

[Woodonglass]

Sooo, if you don't mind, what were your mix ratios for the MEKP and Gelcoat? It sounds like either not enuf MEKP or the MEKP might possibly be bad. It does happen. Putting PVA on it at this point in time is not gunna do much good IMHO. You have a certain window of time to do this and once it's gone it's gone. You'll prolly need to take it off and start over but I'm NOT a GelCoat guru so maybe ondarvr will stop by and lend a hand.

 

[gm280]

If this was purchased via U S Composites and used in the shelf life time they guarantee it to be good, I can't possible see any of it being bad, especially the MEKP! U S Composite is not a fly by night company and they sell tons and tons of such materials weekly (I know) so there is virtually no way for anything to be old or questionable in my opinion. So I would certainly relook at the mixture process and see what went wrong. JMHO!

 

[chieftaing]

Followed US Composite's sheet on MEKP:
1 PINT (16OZ) 1/8 OUNCE
1 QUART (32OZ) 1/4 OUNCE

So that's 1 part hardener, 128 parts gelcoat (they use the same ratio for resin). That's
7.8% by volume.

Temp was quite warm - mid 80's or higher.

Wax was:

1 PINT (16OZ) 1/4 OUNCE
1 QUART (32OZ) 1/2 OUNCE

Styrene thinner was 10% by volume.

From what little I've read about PVA, I agree - too late.

I'll await some more replies.

When you say "take it off" - do you mean all the gelcoat I applied, all the way down to the CSM & resin I used on the new decking? Using sandpaper or a grinder with the 60 (80?) grid wheel that I used to prep for the glass work?

Thanks.

 

[ondarvr]

There can be several reasons for it being tacky on the surface, and the color of the gel coat can make it more likely, dark colors tend to be harder to get tack free.

The catalyst should added at about 2%, which is also 50:1, same as premixed fuel. The wax should be at least that amount or more, up to double that.

Thinning the gel coat with styrene allows the wax to come to the surface sooner, and the wax is dissolved in styrene, so add that amount of styrene too. Sometimes when using a roller if you continue rolling or working the gel coat you can remove the wax from some areas leaving it sticky.

High humidity and low temps can do it too.

If the areas need to be recoated you can just go right over the tacky surface with another layer of GC, no prep is needed.

Sometimes you can just wipe it down with acetone and get down to a tack free layer that can be sanded.

Since you will most likely be sanding most of the final coat off it doesn't make a big difference in how you do it.

Also, you only need to wait about an hour between applying coats of GC, it may vary a bit due to ambient temperature though.

 

[ondarvr]

I forgot to mention.

Wax is dissolved in styrene by melting it, if it gets cold it can solidify again and separate out of the styrene, so while you think you're adding wax, there isn't much in the mix. This can happen before you buy it, so you know nothing about it, warming it up will let the wax dissolve back into solution.

This can also happen if the gel coat is cold, the wax solidifies quickly and doesn't spread out on the surface as it should.

On the other end, if the the temperature is high, or if it's done in direct sunlight, the gel coat cures rapidly and ther's not enough time for the wax to come to the surface.

 

[chieftaing]

Thanks Admiral ondarvr - so here's what I'm thinking...

Unless your recommended hardener ratio of 2% is by weight, then the ratio that came from US Composites is 7.8% which made it harden much faster. I felt like the GC was curing awfully fast, I'm sure the high temps (mid 80s plus) made it worse. Its white GC, applied in a covered garage.

So I think your comment: "On the other end, if the the temperature is high, or if it's done in direct sunlight, the gel coat cures rapidly and ther's not enough time for the wax to come to the surface." is the best explanation for what happened. I didn't re-roll after applying, but I did a lot of rolling just to get good coverage. The brush tipping may also have interfered with the wax sealing the surface.

I didn't stir the bottle of wax, didn't really check it out, so any separation might have contributed to problem.

My plan:
1. Clean off the surface with the new can of Acetone I just bought at Lowest
2. Buy another quart of white GC from US Composites
3. Possibly sand the deck and final wipedown with Acetone
4. Hopefully temps drop a little, so do the final GC when temps are a little lower
5. Use MKEP at 2% by volume (use metric measurements to make the math sane)
6. Use styrene at 10% so it can be sprayed
7. Mix up wax, ensure its warm enough and mixed well enough
8. Use wax at 50% by volume, probably do 75% That's way more than US Composites says to use. Not doubting you, just trying to puzzle it out.
9. Spray final coat
10. Hope for the best.

Anything to add/change? Thanks again.

 

[gm280]

chieftain, My very first polyester resin mixing from U S Composite took three days to cure. And I too followed their explicit directions to the "T". However, their directions basically gave me a 0.5% mixture and with cooler weather, It took three days to cure. So I changed my mixtures to 1% to 2% depending on the temperature and all is well now. I also switched to the metric system and even bought a MEKP measuring bottle marked in milliliters and started using that. So I would hold off doing anything for a few days and see what happens. Who knows it could cure and work out fine after all... JMHO!

 

[ondarvr]

The ratio can be by weight or volume, the amount comes out as slightly different, but well within tolerances.

7.8% on catalyst is so over the top that it could easliy compromise the cure, check their recommended ratio again.

I'm not sure what you mean by 50-75% wax, you would normally add about 2-4%, basically it's just styrene with a small amount of wax in it.

Adding another 10% styrene would would be far too much if you were already adding wax at the high 50% or so ratio.

There are products designed for adding to gel coat so that it will spray and cure better other than styrene, they aren't always available locally though.

PS. If you meant adding wax at 50-75% of what the catalyst ratio is (which should be 2%), then it's closer, and could work, but isn't as reliable as the higher amount.
Retail outlets for gel coat don't make the products they sell, they buy from people like me and then repackage it into a container with their name on it, and while much of their information can be good, they aren't chemists, they just sell the products.

 

[Woodonglass]

Retail outlets for gel coat don't make the products they sell, they buy from people like me and then repackage it into a container with their name on it, and while much of their information can be good, they aren't chemists, they just sell the products.

All iBoats Members should memorize this statement!!! But...That's JMHO

 

[chieftaing]

Sorry about that 50% number I wrote - I was thinking the 50:1 you mentioned but typed the percentage.

I'm attaching the PDF from US Composites. I understand what you're saying about retail outlets; US Composites has been really great to deal with and very helpful.

They say use 1 oz MKEP with 1 gallon (128 oz) resin or gelcoat. That's 128:1. So that's 1/128 = .0078 = oops - that's .78%, not 7.8%. So, my guess that the gelcoat hardened too quickly, not leaving enough time for the wax to rise to the surface is a bad guess. I'm now guessing that rolling on the final coat on a very hot night prevented the wax from rising to the surface the right way.

So I'll stick to the plan and try another final coat with wax. I'll experiment with 2% hardener and wax first on a test panel that I've been using to practice on.

Thanks again.

 

[gm280]

Yes chieftain, That is the same instruction sheet I used as well. And mine took three days to cure using polyester mix. So I switched to metric and now shoot for 1% and 2% mixtures depending on the temperature. And I have not had one problem since. So swap to the metric system and use that percentage mixtures and I think you will like it, especially if you use measuring cups for the efforts. They use milliliters and it is so easy to mix 300 milliliters poly and then 3 milliliters of MEKP for a 1% mixture. JMHO!

 

[chieftaing]

I've cleaned and sanded the deck - used laundry detergent & rinse, sandpaper starting at 180, through wet 220 and will do a final 320. Sanding doesn't really do much, I guess because its in such ratty shape having not cured and using the boat with it still sticky.

Temperatures are cooler in Mass this weekend - so I'm shooting another gelcoat layer without wax, then after a few hours, a layer with wax. I'll go back to using the gun, despite all the cleanup involved. I will use metric ratios. The bottle of wax will get a little additional styrene (since I think just poured out mostly styrene on my first attempt), and will really warm it up and mix it well when adding to the final coat. I'll report back soon. Sure hope this works out.........

 

[ondarvr]

If the surface is uneven with orange peel and maybe some pits or valleys, you can speard some un-thinned gel coat on the surface before spraying or in between coats, use a bondo type spreader to do it, The spreader needs to be a little on the stiff side, so some dry wall tools can work too. This fills the low spots and doesn't add anything to the already high spots, it helps to eliminate a great deal of sanding.

 

[ondarvr]

And when sanding like this, you could have just used the 180 grit and gel coated over that, no need to go finer. Actually you will get a better bond with 180, I normally don't go finer than 80 grit if I'm going to apply gel coat to a surface.