Spraying Gelcoat: Do It Right and Protect Your Vessel for Many Years

Spraying Gelcoat: Do It Right and Protect Your Vessel for Many Years

Gelcoat is essential to maintain your boat's look and sea-worthiness. Gelcoat isn't just a pretty resin that makes the vessel look nice; it protects the fiberglass surface and acts as a barrier to water absorption.

It's like the paint on window frames. Over time, weather fades the paint and it starts to flake and fade. Eventually, the rain soaks into the wood and begins to rot the frames. Your boat is the same! 

Imagine the punishment it takes under the water line day after day. Put simply, gelcoat keeps you afloat.

Can You Spray Gelcoat Over Gelcoat?

The short answer's yes. But first, make sure the surface is free from dirt and debris. Using the window frame analogy again, you wouldn't apply new paint over old paint without preparing the surface. The paint would peel away within months and you'd have rotten frames.

It's advisable to remove as much of the old gelcoat from the hull as possible if it's blistered. Use low-grade sandpaper and scale it up, starting with 40 or 80 grit and finishing with 100 or 200 grit. That should remove debris and rough up the surface enough for new gelcoat to bond.

A power sander will do the trick, taking the blistered area back to the fiberglass. If the old gelcoat is faded but still fine, apply the new coat directly on top. The old gelcoat will act as a perfect substrate and allow the new coat to harden.

Preparing the Boat

Before I get into how's and when's of spraying gelcoat, there are basic housekeeping rules to cover:
  • Avoid applying gelcoat in direct sunlight.
  • Choose your day carefully and watch the weather. Never apply gelcoat in temperatures below 60 degrees F or above 80 degrees F.
  • Allow for ventilation. Never apply gelcoat in an enclosed space.
  • Remove all attachments: cleats, ropes and seals. If necessary, apply duct tape instead of masking tape (it bonds better and gives a better seal).
  • Duct tape around portholes and anything that you don't want in contact with the gelcoat.

Now we can get on with preparing the surface to be gelcoated.

Cleaning and Sanding the Surface

Ever heard "The Devil is in the details?" The tedious prep work is where you succeed or fail in a smooth, well bonded and professional-looking gelcoat.

So take time and get it right. Sanding the surface will smooth out any lumps or bumps. Don't think they won't show through either. When you've finished the gelcoat, all you'll see are areas where you cut corners and adopted a "that'll do" attitude.

After the surface is sanded, run your hand over it to feel its smoothness. Still bumps? Keep sanding!

Making Sure All the Holes and Dents Are Filled

Depending on the dent's size, you may need to fill before sanding. Here are some great products on the market for this job:

Bondo 00277 Glass Reinforced Filler (Check Price on Amazon): This filler sets quickly, enabling sanding within two minutes. It also works on wood, fiberglass and other materials.

Marine-Tex White (Check Price on Amazon): Great for repairs and filling dents and scratches. This filler is versatile too. It bonds to wood, fiberglass, metal and even masonry.

Once you've located and filled all the dents, it's essential to grade up the sandpaper. As before, start with a 40 to 80-grit and then scale up to 100-grit sandpaper. This ensures the feathering is finer, which makes it easier to apply the resin's first coat.

Once set, rub down the filler with high-grade 180 or 200-grit sandpaper to smooth it like a professional. After you're satisfied with the finish, wipe the entire surface with Acetone to remove any grease or dirt. This helps the gelcoat bond better with the surface.

Make Sure the Area Is Well Ventilated

I mentioned using well-ventilated areas to apply gelcoat, but on the flip side, you should also avoid the outdoors. You don't want dust getting onto your gelcoat's fresh new layer.

Speak to marinas and ask about suitable facilities like poly-tunnels and dry docks. Make sure that you wear appropriate face and hand protection too.

Preparing the Gelcoat

Before applying gelcoat, calculate how much you're going to need. One quart of gelcoat, at a 10 to 15 mils thickness, should cover a 14-foot hull. You might need to re-coat areas, but the depth should be no higher than 20 mils.

As a rule of thumb, gelcoat should be prepared in 5 oz batches. Here's why: Although more significant quantities can be mixed, the pot life gets reduced significantly. You'll have roughly 15 to 20 minutes to apply the first coat.

Also, before you add the thinner, you might want to add a sanding aid like FGCI Wax Additive (check price on Amazon). This helps the sanding process once the gelcoat gets added. It helps cure the resin at room temperature and allows a less tacky surface for sanding.

Adding Thinner

Only add thinner when using a gravity-fed spray gun (check price on Amazon). For the professional standard "Cup" gun, the gelcoat can be added thicker. Although the downside is a less smooth finish.

Two types of thinner are commonly used in gelcoat: Reactive thinners and Evaporative thinners. Both do the same job, except reactive thinners mix to become part of the gelcoat, while evaporative thinners dissipate within the resin to leave mostly pure gelcoat.

Adding Hardener

Hardener, or Methyl Ethyl Ketone Peroxide (MEKP), helps the gelcoat set. Here's a rough guide to the amount needed to add to the gelcoat mixture: Assuming you've used a thinning agent like Patch-aid (check price on Amazon), you'll need 0.625cc of MEKP per ounce of gelcoat. This equates to 12 to 18 drops of MEKP per ounce.

Scaling up, you'll need to add 11cc (approximately 165 drops) per quart of gelcoat. If you're mixing a big one-gallon batch of gelcoat, then 40cc or 600 drops will need to be added. 

Remember: You'll only have a 20-minute window (maximum) to apply. After that, the mixture over-catalyzes and appears chalky. And once applied, if the coating's dull and chalky it will not buff up. 

The color it goes on wet is the color it dries. Speaking of color, at this stage in the process, you'll want to add the color pigment (if white isn't for you).

Types of Applicators: Brush Vs. Spray Gun

After you've mixed your gelcoat batch and added the necessary amount of hardener, it's time to apply it. What's the best way to do so?

Brush Application

If the craft or the area you're gelcoating is smaller in size, then brushes are a lot less hassle. I accept there's no substitute for spraying gelcoat finish for ultimate smoothness, but why all that expense when a brush will do? Accept that the finish will be less professional-looking and move on.

On the flip side, you may need to apply two or three coats with the brush to achieve the same thickness as a spray gun. If you plan on coating your boat's entire hull, and it's a big enough job to warrant some investment in the outcome, then choose a spray gun.

Spray Gun Application

You could spend anything from $65 to $1650 on a spray gun (check price on Amazon), but I reckon if you're paying around $130 you're getting a gun that'll do the job just fine.

It's also worth investing in an air compressor (check price on Amazon), especially if the gun doesn't have one built in.

Some do, like the Homeright Finish Max (check price on Amazon), which makes life easier when spraying hard-to-reach areas.

Now, to apply the gelcoat, stand about a foot away (0.30 meters). Work in sweeping motions (taking care not to stall when you reach the end) and make sure you overlap slightly during each application, so no bits get missed.

Once you've completed the job, leave the gelcoat to set over the next four hours. For best results, leave it overnight.

Great Gelcoat Brands

Sea Hawk Gelcoat (check price on Amazon): Premium-grade marine gelcoat for protection against water sanding and the rigors of the sea.

Shulman White Gelcoat Kit (check price on Amazon): This high-grade gelcoat comes with the hardener for convenience.

Total Boat Gelcoat Neutral without Wax (check price on Amazon): This non-sagging and high viscosity formula is easy to apply and has excellent coverage.


After all this hard work and attention to detail, the last thing you want is for the finish to ruin because you failed with the aftercare. Always store the boat overnight in a dust-free environment.

Letting the resin set overnight gives a harder surface and a longer life to the gelcoat. Avoid storing the boat in direct sunlight while it's curing.

Check the surface for any blemishes once the gelcoat is set. You may need to apply another coat. Check carefully for bubbles and blisters and treat these immediately; they're the weakest points of the coating and can allow water absorption into your fiberglass hull. And you don't want that, especially when a well-applied gelcoat can last ten years or longer.


 Boat Soap

Check Price on Amazon - Once you have that shiny hull, keep it looking that way with boat soap. Better Boat's Boat Soap quickly and gently washes away dirt and grime. It's 100% biodegradable and doesn't leave a slippery film. 

Marine boat polish with carnauba wax leaves a high-gloss shine and protects that sleek gelcoat from algae, UV rays and saltwater. For a scratch and lint-free wash, use chamois cleaning cloths for quick touch-ups.  

Gelcoat is essential for the longevity of your boat. It protects the very thing that protects you: The hull!  So, why would you risk everything for an easy life?

Sure, applying and spraying gelcoat isn't the easiest of tasks, and yes, it's pretty messy. But done right, it gives you another ten years of sailing and knowing your hull not only looks good, but it isn't going to weather in the harsh seas.

Happy boat. Happy you!