wanting to apply new gel coat over old gel coat

Discussion in 'Fiberglass and Composite Boat Building' started by midcap, Nov 11, 2014.

  1. midcap
    Joined: Oct 2013
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    midcap Junior Member

    I have a boat done in white gel, and I would like to apply a different color gel coat over the old gel coat. Besides sanding and wiping down before application will I run into any type of snags while doing this. I plan on shooting the new gel out of a 2.5mm gun thinned 10% with styrene. Of course the last layer would have wax in it to get the surface to cure.

    I really like the robustness of gel coat over a marine paint.
     
  2. ondarvr
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    ondarvr Senior Member

    There can be many snags and pitfalls in re-gel coating a hull depending on the size of the boat, your skills and luck.

    What size boat is this?
     
  3. midcap
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    midcap Junior Member

    it's not very large. a 20 ft wellcraft

    I just want to re gel from the water line to the rub rail. Maybe 20 ft on each side by 2 feet tall.
     
  4. ondarvr
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    ondarvr Senior Member

    This is the short list of what to expect.


    For prep work I normally use 80 grit for sanding. The prep work for gel coat is a little easier than for paint because gel coat goes on thicker and will hide many sins. Any spot you don't sand will chip off in the near future, normally in the most visible location on the hull.

    Thinning with Styrene will cause it to yellow sooner, depending on the color and your expectations this may or may not be a problem.

    White is the easiest color, the darker the color the more chance of having issues.

    Porosity can be a problem.

    I hate finish sanding, so doing many laps around a hull with progressively finer grit paper drives me up a wall, plus good sandpaper can be expensive.

    For someone not familiar with spraying gel coat from a pressure pot (which is what you need, a gravity feed or siphon gun won't do it) having the gel coat harden in the equipment is a real and common problem, this means replacing everything that can't be salvaged, $$.

    Did I say I hate sanding?

    One gallon of gel coat covers 80 sq ft at 20 mils, you will need to spray about 30 mils so the orange peel can be sanded out and not risk sanding through in places. This coverage is not counting how much you waste, which could be a considerable amount. You will probably run out of gel coat with about 10% of the spraying left to be done, and need to order more.

    And by the way, you will sand through in spots and need to re-spray those sections. These sections may not match the rest of it and be visible, especially to you.

    The color won't be exactly what you wanted, but you will use it anyhow, and not like it the rest of the time you own the boat.


    The wind will blow hard and a sudden downpour will takes place right when you are about to finish spraying, this will ruin the entire spray job.


    Buffing takes time too, and after you buff one side you will not like the results, there will be too many visible sanding scratches, so you will sand and buff the hull again. You will sand through again and need to re-spray sections....again.


    If you end up with porosity it's a do over, there is no other way to fix it.

    Gel coat isn't designed or formulated to cure tack free on the backside, it does happen to work most of the time though, if it doesn't, it just makes it more difficult.

    Spraying gel coat in direct sunlight can prevent the surface from becoming tack free, another common and frequently unexpected problem.

    The gel coat will start to gel before you can pull all the tape off, this will mess up the tape lines, you will also drop the tape and/or the masking paper on the wet gel coat, requiring more work later to fix it.

    The compressor will malfunction mid spray job, then stop working completely and you won't have a spare. If you do have spare it will be too small and not provide enough air, so you will be slowed down waiting for it to build up pressure. This results in it gelling in the pot, $$$.

    At least once you will drop a full catalyzed pot of gel coat on the ground, it will splatter all over finding the most expensive items in the area. You then need to decide whether clean everything up or continue spraying.

    You will get over spray on your wife’s car if you do it in your driveway, or on someone else’s stuff if you do it at another location.

    You will get gel coat inside your vehicle, you won’t see it until a week later, it will be on the steering wheel, seats and knobs.

    If you do it at home, your wife, kids and neighbors will complain about the odor. And the over spray on their cars.

    A neighbor’s cat will jump on the boat and mess up the wet gel coat, then the cat owners will want to sue you. The cat will also walk on your car leaving tracks that you don’t see until later.

    Someone will drive by on a dusty road, or mow their lawn right next to you while you're spraying, depositing a good amount “stuff” on the wettest area of gel coat.
     
  5. midcap
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    midcap Junior Member

    haha, that sounds like a lot of fun. I may just stick to a top side poly then.:D
     
  6. bigjonny9
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    bigjonny9 Junior Member

    Gelcoat on Gelcoat shouldn't be a problem no matter what colour. As they say its all in the preparation right?. It comes down to how good a spray painter you are and how good the prep..If your crap maybe go for the gelcoat therefore if you get runs and imperfections you can easily sand them and buff them out. But if you back yourself on the gun go 2k as you should get a nice finish straight off the gun (however alot of prep is required and a good spray booth). 2.5mm gun sounds a bit wide maybe try a 1.4 or 1.6 and at %10 you should be good. I would wax all your gel not just the last pass. Gelcoat is pretty forgiving as long as you can handle a little sanding and buffing if it doesnt go on as planned. make sure you put enough on so you can sand it without going through to the old stuff. Depending on how "peely" the gel goes on you can get away with a 800g or higher wet sand and a good buff and it should come up good. just make sure you have a CLEAN environment..No SILICON!!
     
  7. gonzo
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    gonzo Senior Member

    If you thin the gelcoat with Duratec clear, it spray like a regular enamel. I routinely use cheap gravity fed spray guns from Harbor Freight. If the gelcoat hardens before having time to clean it, I simply grab another. Just keep a couple in reserve.
     
  8. midcap
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    midcap Junior Member

    what's the recoat procedures using the duratec? Sanding in between coats?
     
  9. ondarvr
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    ondarvr Senior Member

    Duratec works but....

    The color will yellow much quicker than it will without it, and water resistance is reduced. The up side is that it will spray easier and level better requiring less sanding, it allows it to cure tack free.

    No need for sanding between layers, just don't wait too long. you can mix it at varying ratios, they recommend up to 50%.


    Short term it works well, long term it compromises the quality of the job. If you are matching a color it can be an issue, if the entire part will be sprayed the yellowing may not be as noticeable (it varies by color and mix ratio) unless you pull a sticker or cleat off, then you will see it.
     
  10. midcap
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    midcap Junior Member

    That's good to know. I want to spray sea foam green as the color of the sides. So yellowing probably won't be as big of an issue when compared to spraying white. Another thing I have going for me is that the boat is kept in covered storage out of the sun when not on the water. I am sure the yellowing is partly due to UV exposure?
     
  11. ondarvr
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    ondarvr Senior Member

    The better the boat is protected from UV light the better Duratec will work as an option.

    Sometimes the color is enhanced with Duratec. When less pigment is added the color can look deeper and richer, it may also buff out to a higher gloss. As you reduce the amount of pigment you can see deeper into the finish, this may make it look deeper and richer, and possibly darker. Also pigments don't buff out to a high gloss, resin does, so as you reduce the amount of pigment the gloss can be higher. As you add a clear resin product like Duratec you significantly reduce the amount of pigment in the mix.

    The down side is that gel coat depends almost 100% on pigment loading for UV resistance, if we could reduce the amount of pigment by 2 or 3% and get the same results we would, pigments are expensive (adding Duratec can reduce the pigment loading by 30-50%). But we can easily see any reduction in pigments when we do weathering tests, so we don't.

    There are markets where products like Duratec work very well though, people that make or redo custom go-fast boats love varied and detailed colored schemes. Even though these boats have the potential to yellow quickly outdoors, they are in sunlight only when in use, the rest of the time they stored in well protected locations, so weathering isn't really an issue.
     
  12. midcap
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    midcap Junior Member

    That's how my boat is stored. That and I regularly use a sealant to protect from the elements. I think the duratec is the route i'll go. It seems to me to be the best route since I store the boat indoors.
     
  13. gonzo
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    gonzo Senior Member

    I've done repairs about eight years ago that look fine. I haven't had any problem with water resistance. I think it's just an old wife's tale. The same as about hardness, call the company and they have data on it.
     
  14. ondarvr
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    ondarvr Senior Member

    I've done the testing, real world and the lab, it shows the same results, premature yellowing. Just had a buddy do the roof on his hard top, it was a custom extension on a brand new boat. He asked me about how it would work, I told him it would yellow sooner than the rest of the boat. He decided to do it because it would save time and even if it did yellow it wouldn't be that noticeable. One year later the hard top was a very different color than the rest of the boat.

    Because a few repairs made didn't seem to yellow doesn't mean this isn't the norm. I'm not aware of any builders of new boats that use this product to do repairs in production due the potential weathering problems. I'm not talking about one off builders, I'm talking about production boats. There are many products made by this company that work very well, and this one does too when used in the correct application. I have traveled with the owner of the company and have done schools with their sales/tech person for the West, good people with good products.

    I should add that every color will start to yellow at a different rate, and with some colors you may not notice the color change at all, and with others it may show up in a few months. The ratio a product like this is used at will greatly influence the rate of yellowing also, higher ratios will speed the onset of weathering. The location of the repair also makes a difference, hullsides don't get near the UV exposure the horizontal surfaces do.

    This discussion is sort of like discussing smoking and lung cancer, everybody knows someone that has smoked since they were 14 and is now 85 and appears to be very healthy, yet all the data shows smoking leads to many health issues and commonly premature death.
     

  15. dinoa
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    dinoa Senior Member

    Gelcoat is a lousy paint. Seriously consider PU or urethane acrylic after filling and priming.

    Dino
     
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