Gel-coat problem - Ski refurbishment

Help ! Advice needed.
The gel-coat of my Fenn XT has aged strangely : It looks like it has been cut with a very sharp blade (can be seen on the pics). In some other places, it looks like "orange skin" (does not show well on pics).
If I'm not mistaken gel-coat is polyester resin.
The surfski is made of carbon, so that means epoxy resin.
What would you do to fix it ?

another pic

Hiro-I don't believe polyester resins are compatible with epoxies. I am guessing that your boat has an "epoxy" paint finish (something along the lines of "Imron" ,an automotive epoxy base finish.) I of course defer to the mfg. who I would advise you contact regarding materials used in the finish and procede from there.

It's so hard to sand and so thick, that must be gel-coat.
For what I know, epoxy adhere to polyester, so that's no problem during the building process : spray polyester gel-coat in the mold and then laminate with epoxy.
The problem arise when you have to repair gel-coat : will the polyester gel-coat adhere to the epoxy underneath ?
Or is there some kind of epoxy gel-coat that I don't know of ?
Or do I have to sand all that gel coat and paint the ski ?

Why does carbon mean epoxy ????? I disagree that epoxy resin and polyester gelcoat stick so that is no problem.(it can be a big problem)

There is an issue when using normal styrenated gelcoats that the "sticky" feeling of unreacted inside gelcoat that allows a good chemichal bond to styrenated resins (eg poly/VE) actually stops the mechanical bonding of the epoxy resins

As to those surface problems it may be some paint (rework?) that is not stuck to the same colour Gelcoat due to insufficient preparation. Did you buy it new ? how old is it ?

To fix i suggest sanding the entire surface smooth and repaint with Polyurethane paint(and only use primer if necessary.)

Looks like it has been repaired in the past by somebody that didn't know what they were doing to me.

That second picture sure looks like it has been sprayed to me.

There is a chance there was tiny air voids trapped under the gelcoat in a chop strand mat layer which have expanded to make that "bubbling" but there are too many for my thinking , i think its more likely that the areas have been repainted in the same colours as the original gelcoat and the paint/preparation is insufficient.

ps 2pack LPU (Linear PolyUrethane)paint is often called Epoxy paint as it uses a hardener like epoxy does

unlike catalyst in Polyester or Vinyl ester resins and gelcoats

latman wrote:

To fix i suggest sanding the entire surface smooth and repaint with Polyurethane paint(and only use primer if necessary.)

I will second this as an option to fix it.

Thanks for your answers.
That boat is 4 or 5 years old and I bought it new. So I don't believe it has been painted (unless they had to do it off the mold for cosmetic issues, but I don't think so). I scraped it with a sharp razor-like blade and it's very hard, so that must be gel coat...
I think will scrape/peel/sand everything that can be and use some epoxy filler.

I started to work on it and the more I sanded, the more it became obvious the whole gel-coat must be taken away : small cracks an pinholes everywhere !

Another pic.
And that's definitely gel-coat. There's no paint. Although in some places it looks like there are two layers of gel-coat and that they are separating from each othe. I don't understand how it can be like that...

So I took my sander (see pic).
When the sanding'sover, I think I'll have to glass the hull (with epoxy resin) before I paint it. Do I have to fill holes before I glass or after ? Or maybe it doesn't matter ?
Do I really need to glass the hull or is an epoxy primer OK ? Like the one you use on boats before you paint with antifouling.

latman wrote:
To fix i suggest sanding the entire surface smooth and repaint with Polyurethane paint(and only use primer if necessary.)

Would you please tell me why I should not use primer ? Or what is the criteria to determine if primer is necessary or not ?

The ski is 19' long, 20" wide. How much resin is needed for glassing ? How much paint for 1 coat ?
I won't find those products at the hardware store in Huahine (where I live) and I'll have to order them from a shop in Tahiti so I need to know how much is enough...

And I tought that was a 2 hours job !

If you do not sand to the fibres you can get away without primer IMO which would be lighter and simpler , you would need to fill any pinholes with filler (bondo?) If you were to spray ANY form of paint now you will see any problems (pinholes or fibre print)areas very easily and they will have to be fixed (filled and sanded) before topcoating. You MAY get away with 500ml total of two part paint if it is sprayed thin (and light) remember there may be up to 250ml of two part hardener (and maybe some thinner/reducer too)that will increase the volume you have available. You will likely have to paint the deck (more painting/primering) at a different time so it call all dry whilst sitting on racks/stands. Good luck its a big ugly job

IMO the ski undoubtedly had "issues" that were repaired and gelcoated(flowcoat sprayed) without adequate surface preparation.

I've sanded that gel-coat, with 150 and 220 grit... took me a loooong time !
Applied some epoxy filler where needed and sanded again...
BTW I decided to make it look "seamless". I've always tought the seam between the hull and deck was ugly.

Painted the hull with primer.

Before I contact the builder, please look at that pic, and tell me what you think. This is how the gel-cot came when I sanded. Is it normal that the color is lighter under the glossy surface looks like a differebt shade of blue) or does it shows that the hull has been re-coated or painted after getting off the mould ?

No, can't really say for sure.

However, it is normal for gelcoat to darken in colour with age. My one 'orange' kayak is now almost light brown. This is also one of the pains of fixing boats with gelcoat that is older; it is very difficult to match the colour since the new gelcoat is a much lighter shade.

some more filling

paint : first coat.
Will sand tomorrow and paint again. I need more hours in a day !