How to repair mould gel-coat (lots of photos)

So, I’ve been doing the tidy-up on the mould before I infuse it, and I’ve had a bunch of problems with bubbles behind the gel-coat. These are down to a mistake I made whilst building the damn thing. One puts the gel coat on the mould, or it gets the hose again. Well, I put the gel coat into the mould (spraying), and let it set. Then you put a coupling coat of glass on the back. This is pretty thin (tissue) glass – 100gsm chopped strand. Or you do the wrong thing, and try a new product. I tried a coupling veil, which is like breather fabric. I found it was very very hard to wet out, and hard to make it stick to the gel. What’s more, I also started running out of resin. So, half way down the tub I stopped using it and went back to tissue. When you look at the mould now, you can many more bubbles on one side than the other where I swapped from veil back to tissue.

What I’ve found is that I’ve ended up with a lot of repairs to do to the mould. Here’s my technique – it may work for you.

Step 1 – Find out where the hole is

Once you’ve found it, dig it out with a screwdriver – be sure to probe around the edges – it’s quite surprising how far a run is from a simple small blob.




Step 2 – make it concave

One of the things I’ve worked out is that when getting the gel in the hole for the repair, you can’t get it into every pocket a the edge of the gel .If you’re digging out with a screwdriver you won’t get under every edge, and if you’re coming in down from the top with the screwdriver, you may not lever out the weak points as well where adhesion to material and gel isn’t perfect. So, what I do is put a grinding stone on the Dremmel and go around all the edges until the edge of the hole is convex. Then there’s two advantages – one: no overhanging void that the gel can’t get under and leave a small air pocket you’ll be repairing later; two: I’ve found that it occasionally chips out another void that wasn’t visible at all. It only takes a few seconds to do this. Of course kiddies – safety first. This kicks out a lot of dust and is prone to kicking up gel-flakes, so wear a mask and goggles. It’s also loud so i go for ear defenders. I look like the human fly.

Here it is, all dug out. It’s interesting that I found a lot more void when digging it out.


Step 3 – Apply the gel

Gel-coat is weird stuff, and doesn’t fully set in air – it needs a barrier or it remains tacky. There are two ways to do this – first is add a liquid wax solution to the gel coat (typically at 2%). As it sets, the wax migrates to the surface and forms an air-tight seal. It works if you are doing your repair as a one-off or is perfect if you’re spraying a repair. However, I think there are downsides to this for spot repairs:

  1. If you have to build up a repair, you will have to take off the top layer to get rid of the wax (mould cleaner disolves it away, but you’re using expensive chemicals).
  2. If you are building up a repair, it will leave a relatively smooth surface, so not much of a key
  3. The wax is dissolved in styrene (which should totally gas out though) and epoxy and styrene don’t get along well
  4. It is extra faff adding 2% wax to, say, 30g of gel-coat.

So what I do is make up the repair, and seal it with flash tape. It’s specifically designed for resins not to stick to it, as well as being stable at high temperatures. This works quite well for making deeper spot repairs, and the tape gives the gel-coat lots of support if I’m repairing a vertical surface. If you’re using wax, you have to build it up in multiple layers or else it will run. This method is faster (for me).

Step 4 – Flat it off


I don’t have a picture of the large wound I repaired half-way through, so I’m going with this one. If you run your fingers over the repaired gel, quite frequently you can feel it’s slightly proud. If you can feel it, you certainly will see it on a cosmetic part. Worse, if it’s half a millimetre proud or more, it may give you release issues as well. You can see around the repair that I’ve started to flat it off. I find a 120 grade paper on a small random orbital sander such as this Ryobi. It’s great because it also has a extensible pointy nose thing that gets into the corners. Like a wasps ovipositor. Kind of. Maybe. I just like saying ovipositor.
The white speckles you can see are where the gray top coat has been flatted back to the white undercoat. Once you start seeing white, you stop. The white line at the top of the repair is a slight highlight reflecting the light from the spotlight I’m using. It certainly shows there’s a lip there. What’s more, this was two holes so I knew to keep flitting back until I’d seen the two holes again. It takes about 10 mins and some patience to do it with the Ryobi.

I didn’t want to go any more coarse than 120 grit because I’d be digging deeply into the part. 120 makes good progress, but then flats off well. To flat the part, I used an 800 grit disk on a sander like this one which did it quickly and safely . No gouging. From 800 I went straight to 1500, then onto the polishing compounds.


Here’s the finished version of the holes above, or one similar. What you end up seeing is the original holes filled in. once they’re polished (800 -> 1200 -> 1500 polish ->2000 polish -> anti-swirl polish) the new black gel-coat comes out as shiny as the original.






So, here’s the final result, polished as well. You can even see the little loop in the picture in black where i accidentally scratched the gel coat with the Dremmel. it’s in the third photo from the top. This is a very solid repair and will take multiple pulls if necessary





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