Annoying defects in gelcoat repairs.

I've been doing gelcoat repairs for about 4 years now as part of my part time job, but it seems no matter how many come out near perfect, I still get the odd defect crop up...

1. Most common; Bubbles. You mix up the gel, with wax and catalyst, stir it gently to make sure no bubles form when you mix it, and then you put it on the repair and let it go off. when you come to fair it back the surface looks fine but under that is a bunch of pin-holes that you have to go back and fill again...why! doesnt happen all the time, doesn't seem to matter what the temperature is or if it had more or less catalyst or wax in there (i admit to not being super accurate, but i thought it was fairly non critical? 1-7% being ok?), seems to be pretty random.

2. And most annoying; Discoulouration. We did a large job on the side of a Hunter 707, looked near perfect when it left the shed. 9 months later and its gone grey (was 'Ice White' before..white with a very slight hint of blue). Is that the gel? Too little wax? Imperfect polish finish? The gel came from the builder.

3. Got a drum of white gel of unknown age, used it to flow coat a test tank for outboards, Its still very, very slightly tacky 2 months later. It was done in two seperate batches so I Know I didn't screw up, and not put any cat or wax in it cos' I doubt I would have screwed up twice! Looks fine in the drum... still flows well, so no lack of styrene.. does gel die after a while?

I've seen countless crap repairs, but also i used to work with a guy who seemed to make a perfect job every time, so it must be possible! The worst (and best) thing is often an owner can't spot it even when i think i've really messed up, but then i see the boat on the marina later and cringe!

 

Lots of good questions ML

When I teach repair classes this question comes up almost every time, "why do some repairs turn out great and some don't?"

One problem with polyesters is they are so forgiving that people get lazy in measuring and application techniques and they still work most of the time. Then it seems without warning you get bad results.

When doing repairs it is critical to follow the directions and accurately measure all ingredients, there are chemical reactions that need to take place that rely on the correct amount of each product being present. It is even more important for it to be correct on a repair, because polyesters do not cure well in a thin film. Gel coat should be 18 to 20 mils thick (wet) and warm (77F+) to cure well, many times it's not that thick or warm on a repair so it needs all the help it can get. So measure it and use the same blend every time.

1. Are you spraying or brushing the gel coat on when you find pin holes?
If you are spraying the gel coat, tip size, air pressure, how close you are and technique can create problems. Large air bubbles can be caused by a large tip and too much applied at one time, or being too close and pushing the gel coat. Even on a warm day the wax can start to solidify when being sprayed, especailly if acetone is used to thin the gel coat. Acetone will rapidly cool the mix when it passes through the nozzle, the wax starts to form chunks instantly, these chunks hit the wet gel coat on the surface and create pits and bubbles.

Catalyst type can also create porosity, so make sure it's fresh and use the type recommended by the supplier.

If you're brushing it on, again put less on at a time, most gel coats are not formulated to be brushed and may not release air well when used this way.

2. Every batch of gel coat (or any type of paint) will be slightly different in color, it may or may not be noticable, but it is, it may also weather differently.
If possible all repairs should be done with the same batch of gel coat used to make the boat, this isn't always possible so you just do the best you can. Many times you don't even know what brand or type of gel coat was used to make it in the first place.

What you need to do is again measure everything, catalyze at 2%, if you need to thin the gel coat down don't use acetone or styrene, they can change the color and may make the repaired area weather differently than the surrounding unreduced gel coat. Styrene yellows rapidly in sunlight, so the more you add, the faster and more it may discolor. Acetone can change the color of some gel coats and may cause curing problems. I know everybody uses them, so do I at times, but they can create problems.
There are patchaid type products that can be used to thin and promote gel coat so it will cure faster and better in a thin film, some have wax in them so you won't need to add it, or spray PVA over the repair. Only add enough to make the gel coat spray well, this is normally around 20%. These products may change the color of the gel coat too, it just depends on the color, so add as little as you can get away with. Once you find the correct amount for the type and color of gel coat you are using, always measure and use that exact amount.

3. Old gel coat may not cure well, it may look good ,but the gel time or other specs may have drifted.

If it's not cured by now, it never will.

Measure accurately, it makes a big difference.

Gel coats are not designed or formulated for post coat type applications, they do work most of the time, but I have used gel coats that you cannot get to cure well on the backside no matter what you do. There are no QC tests that gel coat needs to pass to ensure it will cure completely on the backside even when wax is added.
The type of wax used can also be different from one supplier to the next. The wax needs to come to the surface to seal it from the air, different waxes melt and then solidify at different temperatures so you may get different results with the different waxes used by suppliers when the tempeature changes.

Styrene is heavier than air, so if there is no air circulating in the tank, the styrene will settle and inhibit the cure. Once the styrene is removed it should cure, but not always.

If the gel coat you used is one of the newer low emission products, it may be even more difficult to get a good backside cure. With less styrene they just don't cure as well when used this way.

If you apply waxed gel coat in direct sunlight it will normally not cure well on the surface, so to be safe, do it in the shade.


I just looked at where you are located, brushing is common in England and I assume thats how you are applying it. Gel coats made there are formulated to be brushed, but all of the same rules apply.

 

I had a feeling accuracy had somthing to do with it, but what you say about temperature is true as most of the time I'm doing these repairs outside in the great British weather. I didn't realise that sun had an effect (not that we get much!). Time to get a better set of scales.

Most of my repairs are brushed or layed on with a mixing stick then quickly covered with masking tape to keep the gel in place. What you've said about thin coats explains why using a foam roller and putting it on like paint doesn't work. Might be getting a spray gun soon. The products I use are a mix, mainly Scot Bader white and clear gels and catalyst but with gels from the builder for some one-designs we have fleets of in the yard.

I have a feeling the wax i have may be slightly iffy, It solidifies in the despenser bottle tube frequently, surely its just wax and styrene so nothing could go too wrong? Too little styrene?

Also, where I currently work some of the guys use 'pre-gel' (like a really thick, clear gel) to thicken gelcoat to help keep it in place. I was taught to do repairs without it so I dont tend to use it, and I also find it gives a poor finish when buffed up. What aplications would you use it?

Thanks very much for your reply, very few people in this game, good or bad, seem know why things work or not, and everyone seems to have their own methods. Hope you dont mind my second load of questions!

 

When a repair is covered immediately after the get coat is applied, there may not be enough time for the air to come to the surface and escape, so if it's covered too soon it may trap air in it. Also if the gel coat is thickened, it will slow down or stop the air from escaping. High humidity as well as cool temps can increase the viscosity of some (most) gel coats, so this will slow the movement of air too.

The wax will separate from the styrene at low temperatures and solidify, so it needs to be kept warm. If it's added to cold gel coat it will do the same thing, so make sure the gel coat is warm also. Mixing the wax in with a drill motor will do a better job than mixing it by hand with a stick or putty knife in cool weather.

Anytime you add a clear product to gel coat it will change the look of it.
If the gel coat is 20% white pigment and you add 25% of a clear product (gel)
to it, you just reduced the pigment loading. With less pigment, the color of the base resin will influence the final color to a higher degree. Some of the additives have a hint of color themselves, so it may change it even more.
As you have noticed, laminating resin changes color dramatically after it has been catalyzed and then eventually cures, most gel coat base resins do the same thing, only to a lesser degree, the pigment hides the change in color.

This is what I do if I'm not going to spray the repair and need to get it done fast. This is after any glass repairs have been done, or if only cosmetic repairs are needed and it's larger than just a small air bubble.
Fill the uneven or pitted area with gel coat putty, if the area is small, and I can get the putty on very smooth, then I don't wait for it to cure so it can be sanded, I only wait until it's gelled. Then I use a putty knife or squeeggee to spread gel coat over the top of the putty, this will force gel coat into all of the pits or low spots left in the putty and not add gel coat to the high spots. I will wait until this starts to gel, then brush gel coat with wax added over the whole area and do the best I can to get it smooth. Wait for it to cure completely then sand and buff. Any of these steps can be repeated as needed to get the desired result.

If you are going to spray it, then many times you may not need to squeeggee gel coat over the putty it if you can it smooth enough. If you can get it smooth enough, then you don't even need to wait for the putty to gel, just spray over it. If you do need to use a Squeeggee to fill minor imperfections, you will need to let the putty gel. After you squeeggee it, you don't need to wait for that layer to gel, just spray over it. Wait for it to cure, then sand and buff.

Depending on the type gel coat and if you use a Patchaid type product, this whole process can take less than 45 minutes from start to finish.