Subj: Filling old holes and touching up Gelcoat (Boat repair and maintenance)
Date: 4/22/99 10:32:52 AM Pacific Daylight Time
From: Judy Blumhorst
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Hi Webgang,

A few folks have written me to ask about filling holes and touchup for the gelcoat on the boat when they move rigging hardware. So here's how I do it.

Judy B.

If these are small holes, under 1/2", I'd do it one of two ways. Either one is good, it depends on what materials you have at hand.

In both methods, first use clean the area to be repaired as follows: Use a drill bit to remove any remaining bedding compound inside the hole. Use a razor to remove any bedding compound on the surface. You may need to use laquer thinner or acetone as well to clean the area well. Don't use WD40 (which works well to remove gook 'n putty on gelcoat) because it might get into the core and mess up your repair.

Method 1:

Materials --

-polyester filler with cream hardener (like Evercoat Formula 27, sold in hardware stores, marine stores),

-two or three plastic spatulas, (available in automotive stores, hardware stores, marine stores. You'll find it right near the filler) ( I buy mine at Tap Plastic. They're very flexible, yellow polyethelyene(?) (like tupperware). They're about 6" long, double sided and easier to clean with acetone than the ones you find in hardware stores. I cut them down to about 3" for small jobs like this. Clean them with acetone immediately after each use.)


-disposable gloves to protect your hands (acetone eats the white latex ones so wear two, or buy the blue ones from an automotive store or marine store)

-small wood mixing sticks, (tongue depressor, Popsicle sticks. Cut off one curved end to make it square for better mixing and scraping.)

-several pieces of smooth cardboard for mixing surface for filler putty (or I use a clipboard with a large ziplock baggy opened up and caught under the clip. That way I can quickly change the bag on the clipboard for each new batch of filler or putty)

-gelcoat from IM ($5/half pint) and liquid catalyst (methyethyl ketone peroxide, I think ) or gelcoat patch kit with Light Sienna coloring agent from hardware store. Sienna Gold is used to "age" white gelcoat; when white gelcoat is exposed for years to UV rays, it gets a golden brown/mustardy tint to it. I use Sienna tint from Fiberglass Hawaii in Santa Cruz, California. (831-476-7464)

-paper towels, rags.

-three sheets each of wet/dry sandpaper 300, 600, 1200 grit (available in good hardware stores or automotive stores).

-Unwaxed paper cups make good mixing containers for gelcoat.

-7-day blue masking tape from 3M (not the cheap stuff, buy the good tape)

Method 1, Step 1: Filling> the hole with filler putty

Tape the underside of the hole.

Use water resistant polyester filler to fill the hole as follows: Mix up the filler according to directions. Using a plastic spatula, mix the creamer hardener in well according to directions, working it on the cardboard until the color is even and there are no air bubbles. Use a plastic putty spatula to work the putty into the hole in several directions to be sure you've really got enough in there. On your last pass with the spatula, get every last bit of putty off the deck. Take a cloth dampened in acetone and gently wipe off the surrounding deck area and just a little out of the hole to make the surface recessed 1/32-1/16" for putting gelcoat in later.

[ First, practice using the putty on a scrap of wood with a hole drilled into to get a feel for the putty. Pick a time of day that's under 75 degrees or the putty will set up too fast, under 5 minutes. If it's hot, you can reduce the amount of hardener to a little more than half the recommended amount. . If the putty starts to set up before you're done, QUIT NOW. One more pass with the spatula and you'll pull all the putty out of the hole. You'll just make more work for yourself. If you do mess up, wipe as much as you can off with acetone on a rag and start over]

Once the putty has set up (usually about 20 minutes is plenty, this stuff sets up REALLY fast), you can apply the topcoat of gelcoat to the hole

Method 1, Step Two: Applying the top gelcoat from the kit as follows (see below for instructions on matching color):

Follow directions on the gelcoat patch kit and put gelcoat on the top of the patched hole. After it's fully cured, wet sand (with a block) using the 300, then 600, maybe 1200. Wax.

Or if you order gelcoat from IM do the following: Spoon 3 tablespoons (one oz) of gelcoat into a paper cup. Let it sit in the sun for about 15 minutes (more or less) to evaporate the solvents and thicken to the consistency of ketchup or mayonaise. Then add 12-15 drops (less if it's over 75 dgrees) of liquid catalyst (methylethyl ketone peroxide, you can buy it at any marine store, automotive store, or good hardware store) and mix well for 60 seconds. Scrape the sides and corners and bottom of the cup to mix the catalyst in well. Transfer the gelcoat to a new paper cup and throw away the mixing stick.

Using a new mixing stick, put a dab of gelcoat on the hole. Over fill it a little, since gelcoat shrinks. Let it cure. When it's hardened to a hard rubbery consistency (somewhere between 5 min and 1 hour, depending on temperature), use a razorblade to trim it flat, but leave it a little high.

When it's fully cured (when the leftover gelcoat in the pot is really hard or at least 8 hours or longer if its cool out), wet sand with 300, 600, 1200 grit.  Wax if you like.

You may need to do the gelcoat thing a second time to get it perfect, since the gelcoat shrinks. On areas that aren't non-skid, you can spread the gelcoat with the spatula 1-2 inches past the edges of the hole and then sand it flat (instead of just filling the hole)

By the way #1, the gelcoat from IM has a surfacing agent in it (as of the time of writing this; double check with the factory for any change in they way they ship it), so you don't need to worry about sealing it off from the air to get it to cure.  If you buy your own gelcoat from somewhere else, you may need to do one of the following a) add surfacing agent or wax, (b) cover it with mylar or acetate film so it cures or (c) spray it with PVA (mist it lightly so you don't make air-pressure marks in it).  (c) I've used surfacing agent and PVA successfully.  Mylar or acetate tends to mark the finish, but you can always wetsand the marks out.

By the way #2 -- the polyester filler usually comes with red cream hardener. That's good because you can be sure you've mixed it completely with the grey filler when there are no more red streaks in it. However, if you can get white cream hardener, your repair will show less. Just be VERY sure you mix the cream hardener in very well or else the filler won't cure.

Method #2

This is how I do it. It makes the repair (almost) 100% invisible if your gelcoat color matches well. This method uses thickened gelcoat instead of filling putty to fill the hole.

Additional materials needed: Silica thickener(from West System, Colloidal Silica #406. Generically also called Cab-0-sil).  This stuff is white, which makes it easier to match the color on a white deck.

Put 1 or 2 oz gelcoat in a mixing cup. Add liquid catalyst (10-15 drops per oz) and mix well. Add colloidal silica thickener until it's the consistency of warm peanut butter (the creamy style, not the crunchy style <g>).  It will be an off white color.

Fill the hole, following the directions as described above for putty. At the end, use a smoth clean rag (old t-shirt) wetted with aceton to remove 1/32" off the top ( you want the hole to be not quite full so you can put in gelcoat as described above.) Let it cure for 4 hours at 70 degrees, or longer if it's cooler. Aim a shop light at it to warm it if it's cold)

Put the final coat of gelcoat over it, as described above, using the evaporated gelcoat from IM.  Remember, you must seal the top layer of gelcoat from the air so that it cures, by adding surfacing agent or spraying with PVA.

Wet sand with 300, 600, (maybe 1200).

Color matching:

If you don't get your gelcoat from IM, you'll need to to check the color match on the gel coat. Here's how to do that

Put about 3-4 oz of gelcoat into a paper cup, or enough to do the whole job. Add the tiniest amount of Sienna you can (just a tiny drop) and and mix well. The coloring agent is REALLY concentrated so add it just a bit at a time.

Don't catalyze it yet! Put a drop of UNcatalyzed gelcoat on the hull and smear it around with your finger to check the color. Add more white or sienna to change the color. When you have the color right, wipe it off the boat with acetone. Now you're ready to get to work.

That's a lot of instructions. Hope they're not too confusing. It's not as hard as it sounds. Read the directions carefully on the filler can and the gelcoat repair kit. If you buy the repair kit, use just a dab of sienna coloring agent to tone down the base white gelcoat.

One last reminder: Read the directions on the cans at least three times before you start. Practice first on a piece of wood to get the hang of it. Make your mistakes on the wood, not your boat.

Additional notes (2004)

BTW, spraying gelcoat isn't hard to do either, but that's a whole 'nuther topic. Gelcoat is very forgiving, because you can sand out any drips or hangers by wetsanding. In 1997 or 1998, I removed all the bottom paint from Redwing's hull and resprayed it using a professional spray gun. A few years later (2002?), I replaced the boarding ladder and the outboard bracket on Redwing, so I resprayed the whole transom panel.

In 1999, I resprayed the transom on a friend's boat with a Preval. It wasn't as good as my spray gun, but after a bit of elbow grease with wet sand paper, the transom on his boat loks like new!

In both cases, prep work is critical to good results. YOu have to be sure to remove all grease, wax and silicone, or else the gelcoat will not adhere. I would use something like Interlux Solvent Wash 202 or FiberGlass Surface Prep to be sure the surface was clean. If the surface has contaminants before you sand, they'll get sanded into the gelcoat.

The sand the whole area with 220 grit paper on a block, taking care not to gouge. When you're finished, rinse the area with water. THE WATER SHOULD SHEET SMOOTHLY ON THE SURFACE, WITH NO BEADING OR FISHEYES. Dry the area with clean paper towels and you're ready to go.

Important: Failure to thorougly clean and prep the area will cause the new gelcoat to chip and flake off!!!

Read the manufacturers instructions for how to thin the gelcoat, and how to catalyse it for the most durable lasting results. (For traditional polyester gelcoat, I personally prefer using styrene monomer as thinner (an active thinner which promotes crosslink bonding), rather than using acetone (a passive thinner which reduces crosslink binding).

Be sure to clean the gun and pot with acetone immediately after use.

Follow all safety precautions -- in particular, use a high quality respirator with brand new organic fumes filters while spraying. I prefer a full face respirator (cost: about $250) because it protects the eyes too (where fumes can be absorbed rapidly) Be sure the respirator fits properly or you'll get one hell of a head ache (and damage your liver). Use nitrile gloves when using acetone, because it can be absorbed via the skin. .

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