Applying Gelcoat

Getting to the point of applying your color matched gelcoat is one of the last steps in finishing off your repair(s). In the video I mentioned that there were 2 common ways to do this; spraying or brushing..

While brushing the gelcoat has it’s advantages, it isn’t always the best approach. For small patches such as chips, dings, scratches and small repairs I believe that it is the most efficient both in prep time and application. However in situations where the repair spot is much larger than what is shown in the video, spraying is definitely the way to go.

Looking at this option a little further, lets go over some of the options for equipment. The first thing that will be needed regardless of the type of spray gun used (we’ll cover this in a bit), is a large compressor. At a minimum a 3hp unit will be needed with a 60 gallon tank. This will provide most of the air needed with brief pauses to let the pressure build back up. Ideally, a 5hp unit will provide everything that you’ll require and then some; the compressor will be able to ‘catch up’ with the demand and occasionally turn off. 3HP units will run constantly and can tend to get a little ‘warm’ πŸ™‚ However, they do work! It’s what I have and so far (knock on wood) haven’t run into any issues.

With that part of the equation in place, now it’s time to talk about guns. Keeping in mind that gelcoat is a resin that has a relatively short working time before it starts to set up (normally 10 min depending on the % catalyst used) I like to use guns that can be broken down quickly. For the most part, this boils down to gravity style guns and cup guns (aka dump guns).

Pictured below is an example of a cup gun. These types of units are an ‘industry standard’ for higher volume application needs. This particular unit is the ES-100, I believe the newer model is the ES-120.

ES-100 Cup Gun

Advantages of this type of gun are:
– The gelcoat can be applied with very little to no thinning
– Simple design which allows quick break down for cleaning
– It’s able to “dump” a lot of material quickly and efficiently

In my opinion, the most attractive benefit of this is that the gelcoat does not require thinning. Whenever a material is thinned it displaces it’s properties. By this I am referring to UV-absorbers and pigments / color. A material that has been reduced will require a thicker film for the same level of protection versus un-thinned applications.Β  Also, when spraying laminating gelcoat (no wax additive), use of PVA will be required to achieve a full cure.

This being said, nothing is perfect. The downside of spraying thick material is that it will not give as smooth of a finish out of the gun, requiring more wetsanding to get the desired appearance.

The other style of gun mentioned is a typical gravity fed gun such as that shown here:

Gravity Gun

Use of this type of sprayer will require thinning the gelcoat to get proper atomization. These units are typically less expensive and can be viewed as semi-disposable. If the gelcoat happens to set up in the gun before cleaning, they can be replaced for $15-$20. Note: These cheap units will NOT provide a good finish with paints! A good paint gun can easily cost $500 or more πŸ™‚

When we start to look at thinners (or more accurately gelcoat additives) there are two brands that I’ve used with good results. Duratec Clear High Gloss Additive, and Patch-Aid. These are both what are considered to be ‘reactive thinners’ rather than evaporative thinners. The difference being that reactive thinners actually become part of the gelcoat and cure together as one film. Evaporative thinners are as they sound, the solvent evaporates from the gelcoat leaving behind mostly straight gelcoat. Styrene and acetone would be examples of evaporative thinners. A lot of Pro’s use these types of thinners, however in my opinion they are more toxic to work with and not as ‘user friendly’. Personally I like to use additives such as the Duratec or Patch-aid.

Use of these additives does change the way that the overall mix needs to be catalyzed. Normal gelcoat should be catalyzed at around 1% – 1.25% Mek-P. When using these additives, the overall mix needs to be catalyzed at 2% or higher (it will be stated on the product container). Also, when these products are mixed in, no PVA is required to achieve a full cure.. That’s one of the benefits of these additives, and for this feature alone, I feel it makes them a very user friendly way to spray gelcoat for the DIY.. Also, because the material being sprayed has better atomization, it will provide a smoother finish out of the gun, requiring less wetsanding πŸ™‚

HERE is a mixing chart showing the ratio’s for the various % of Mek-P courtesy of one of my local vendors, Express Composites.

30 Responses to “Applying Gelcoat”
  1. pettyofficerp says:


    I restore boats back to their glory for fun! LOL, does that mean I am crazy? Probably.
    Anyways, I love Duratec/Hawkeye products. I have used gallons and gallons of their products.
    I wish they where better at educating their customers on how to more efficiently use Duratec products.
    My current project is a Tartan 34 and would like to add a gelcoat design for visual interest.
    Do you have any thoughts on what is the easiest and or the best way to accomplish this mission?

    Keep the vids coming, they are great!

  2. Tim Wakefield says:

    Great help, Thanks alot

  3. Brad says:


    I had a question. Is there any household item that can be used to cure laminating resin or gel coat. I can’t get pva shipped here and the Surfacing agent I just bought was old.

    Do you know of anything?

    • ~Andy ~Andy says:

      Surfacing wax shouldn’t ever go old; other options yes but wax should be good for many years after production. I’d maybe look more at the age of the catalyst or resin. Those are only good for about a year. Other problem may be temps your working in. Poly resins need to be in 65F or warmer during the cure cycle which could last up to 24 hours…

  4. Baldimar Rios says:

    Andy, I wanted to ask you about some questions that keep me concerned a lot. These questions have to do when someone sprays gel coat. I have read your comments above and you say not to add a WAX ADDITIVE to your gel coat, that you use Duratec Clear Gloss Additive. The company that makes Duratec also says not to use wax in the mix. But the place where I am going to buy my gel coat recommends using Duratec Clear Gloss Additive and using STYRENE WAX for work done with gel coat below the waterline. Now my other question has to do with temperature and gel coat. I live close to the gates of hell in the summer time, Las Vegas Nevada were it’s a cool 100-105 degrees in the shade. Would you still catalyze gel coat at 2 percent when using Duratec, or would you recommend me to catalyze at a lower percent because of the higher temp. I hope you have the time to answer my questions.

    • ~Andy ~Andy says:

      Hi Baldimar,

      First thing to remember is that Duratec is not to be used below the waterline; ever…. It is for above waterline applications only. In those situations it’s best to use straight laminating gel thinned with styrene to help with the spray. After it has set up, then go over that with PVA (poly vinyl alcohol). Evercoat is one company that makes this and I’m guessing that the composite store you’re buying materials from carries this.

      ** At no point should wax be mixed with duratec; it will mess up the cure..

      For your project, use the gel as I described above, catalyze at 1% and work in smaller batches so you can empty the material before it sets up in the gun. At no point should you under-catalyze resins for the sake of longer working times; you’ll run the risk of it not curing at all :-O

      Hope this helps!

      • Jimmy Spatz says:

        Hi Andy! Love the site all the informative videos You rock!
        I’m about to spray the exterior transom on my boat with new gel. As an alternative to PVA, can I achieve the same desired results using my layup coats with laminating gel, and as a final coat using waxed gelcoat with the wax already mixed in? I own both types of gelcoat, waxed and un-waxed, and do not own any PVA, so this method should also work for me too correct? Will thinning the waxed type gel with styrene cause any issues for a fully hardened cure for use below waterline?


        • ~Andy ~Andy says:

          Nope πŸ™‚ The styrene will evaporate out and not be an issue. Using waxed gel for the last coat will accomplish the same as using PVA.. Good luck!

  5. Greg says:

    You had four coats total?
    Did you mix a new batch for each coat?
    I just applied a coat of gelcoat for the first time.
    It set up far too quickly for me to apply four coats, especially if allowing for some “set up” time between each coat.
    It is new material from Express Composites. I carefully measured 2% MEK-P with a syringe, per their instructions and stirred for over a minute. It was not a hot day. I don’t know where I could have gone wrong. Even for the one coat I applied, it was hard to avoid brush strokes. Yes, I did the ‘criss-cross’ πŸ˜‰

    • ~Andy ~Andy says:

      Each coat / batch was mixed new, and I only catalyzed what I needed for each coat to avoid waste. Temp plays a big factor in set up time, as does the thickness of the mix in the bucket. For example, 4oz sitting in a cup will set up much faster than 4oz spread thin in a paint tray. Brush strokes are going to be there no matter what you do. The key is to have enough material laid up so that you are able to sand them out without sanding through πŸ™‚

  6. Hey Andy,

    Awesome videos bro! Spent my winter evenings education myself with your wealth of knowledge.

    I took on a project boat last fall 13′ Whaler and I’ve sanded grinded and laid up new fiberglass and faired the whole thing. So I’m finally at the new application of gelcoat stage (recoating the entire hull) and I’d like your advice.
    There’s lots of original gelcoat on the hull but none in the repaired places…would you apply non waxed gelcoat on those areas to build up to existing thickness and then put a waxed coat on top of everything?

    Or what would you do?

    And… how long do you usually wait between coats of gelcoat?

    Can’t wait to hear from you!



    • ~Andy ~Andy says:

      Hi Howard,

      Honestly with the amount of area you’re looking to coat, I think I’d be leaning towards paint :-). If you’re limited to rolling the gelcoat I’d definitely be looking at paint! Gelcoat is going to end up being 5x the work with little benefit. I believe Pettit and Jamestown make paint colors that are nearly original to the Whaler colors. I’ve not used Jamestowns paint, but can vouch for Pettit ez-poxy. Great stuff and can be rolled / tipped and look awesome! Hope this helps!

  7. mike says:

    I’m making some non structural repairs in 3 of my hatch lids. Namely grinding back and filling the holes where the latch/lock mechanism are. The holes were oval in shape, and measured approx 1 1/2 inches by 3 1/2 inches. I’ve had excellent results thus far thanks to your “Hole in my boat” series when it comes to the resin/cloth/CSM portion of the repair. Unfortunately, I have had mixed results with the gel coat portion. I purchased some laminating gel coat and PVA from West Marine last week, and sometimes the gel coat will cure and sometimes it wont. Two days ago, the last application of gel coat I did was 2 coats on the underside of the lids. I believe I had the MEKP right( 10 drops per ounce), and I waited a whole hour before spraying the PVA. This morning I washed away the PVA and let it dry all day. However, this evening the gel coat felt soft so I tested it with some acetone, and it came right off? The only other variable I can think of is that it was raining the day I applied the gel coat. Any ideas? Thank you

    • ~Andy ~Andy says:

      I’m guessing the gelcoat is old. I’ve not had good luck with resins from west marine. I always buy from a composite retailer as their turn-over is more frequent. Most resins have a shelf life of a year if properly stored…

  8. Kurtis says:

    Hi Andy,

    I’m restoring my 1959 Glasspar Trident and I’m wondering about catalyzing gelcoat when thinning it with styrene for a spray gun. I know that the thinning ratio is between 10-15% maximum, but do I thin and then catalyze? or catalyze, then thin. If I thin and then catalyze, do I take into account the extra volume from the thinner when adding MEKP, or do I catalyze based solely on the volume of gelcoat without the thinner?

  9. Stan says:

    Hi Andy,
    I want to shoot white gelcoat over a small area of maroon on my glastron, to redefine a few areas to my liking (long story). Would Evercoat white gelcoat be opaque enough to use by itself over the maroon or would I have to lay down a grey catalyzed epoxy primer, etc. as a base coat of sorts. I’ve sprayed tan gelcoat before and I didn’t use primer but I was simply overcoating cracked tan gelcoat.

    Thanks much & great videos

  10. Clint olson says:

    Hello Andy love your vids.

    I have a question on my restore project. It is a c/hawk pilot house. The inside of the house is what I would call rough finish. It’s raw fiberglass with gel coat on it. The question is how should I peep this to roll on a fresh coat. There is really no way to sand it very well because of the rough finish. Any advice?? Thanks Clint

  11. Clint olson says:

    Sent you an email thank you

  12. Jason says:

    Greetings Andy
    I’ve learned so much from your videos…. I’m doing a large restoration on a 91 mastercraft prostar, the gel coat has seen better days ( crazing, numerous chips, pulled through fasteners) I would like to spray a fresh gel coat on the cap. What do you recommend? I know I will be in for 4 stages of sanding then compound, my big concerns are appearance, durability and repairablity.

    Thank you


    • ~Andy ~Andy says:

      Hi Jason! What kind of equipment do you have available for spraying? When you have a chance please let me know!!

      Thank you πŸ™‚

  13. Brian says:

    I have built a bench seat for the house of my boat. glassed the whole thing faired it out and now am ready to finish. It is about 2ft x 8 ft with this much surface area do you thing i would be better off using epoxy paint or gel coat. I will let you know that I used the west system epoxy to lay up the glass.

  14. David Fletcher says:

    I will be repairing an area of the hull at and along the waterline of my 40 ft sailboat. It is a long story however the area will faired with 3 m vinyl Ester compound and smooth. To get the best gelcoat top surface should it be sprayed and is so how should it be thinned. The oem gelcoat is .25 to .3p thick. How thick should it be to be due water at or near the aera5

  15. Tony Wells says:

    Hi Andy, I live in the UK and am restoring a 17ft Proline center console. The hull is in v good condition as is the walkaround area on the boat apart from lots of small holes that have been filled badly from where attachments were fitted. These have all been dug out and repaired. I am now looking at either painting or re gelcoating the whole inner part of the boat. Any advice would be appreciated. I can spray either.

  16. jay says:

    what about the 3m accuspray gelcoat gun with plastic tip,,,,,,,,and liner,,,,,,,,does it have to be thinned,,,,its seems to be perfect and you dont address this , im am curious as to why?

    • ~Andy ~Andy says:

      I’ve not used that gun. Most people are not equipped for spraying so I don’t focus too much on that. I try and keep the info and tools within reason for most DIY’ers πŸ™‚

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