Why is my paint cracking?

My epoxy paint is cracking mostly around the waterline (below and above). Rust stain is seeping through the cracks.

In 1999 I brushed 11 coats of epoxy (International and Hempel tar epoxy below WL, Epigrip and Interguard above) and a 2 coats of Hempel chlorinated rubber on top within a 3 weeks period on top of a sandblasted and epoxy primed new hull.

The boat is 35' long and was in saltwater 2 years (2001-2003) after first launching, then 6 months (2006) and then again 15 months (2009-2010). We traveled about 15000 miles with it. The rest of the time the boat has been stored ashore, most of that time under cover. The temperatures vary here between +30° and -20°C.

Is it possible that the paint film is cracking because of the temperature differences? Then why would it crack mostly around the waterline?

The paint has not cracked on the bottom plates which are less exposed to the sunlight. Could the light have the power to crack many coats of epoxy paint? Can it be the combination of UV exposure and the splash zone?
I understand that epoxy degrade in the light but does the light degrade through the paint?

The underwater area of the hull has received 2 coats of antifouling paint every time the boat went into the water, 6 in total.
I am at the moment repairing all the rust spots sanding back to bare steel and recoating with Amerlock 400.

Any leads to understand the problem are welcome.

 

I am an epoxy vendor and in the industry for 20 years. Afraid I haven't a clue. coal tar epoxies have plenty of flex. Don't know the properties of the other epoxies. Often on hulls cracked coatings or disbonded epoxies are caused by hull flexing with semi brittle epoxies.

I assume you painted over the epoxy so probably not UV.
still the coatings are 13 years old - not that it means much either, but in your case water temp and air temp very different so it might maybe, be related to hot cold contractions etc. in an old epoxy.

the 11 coats of epoxy seems strange, with most epoxies that would be about (wild guess) 3/8 - 1/2 thick!

paul
progressive epoxy polymers

 

What seems strange: the 11 coats? Thickness is about 2mm I would say looking at the craters I am doing with the sander.
I brushed a total of about 400l of paint on 124 m² of steel plates.

0.4m³/124m²=3.2mm, minus the solvent, 2.5 mm is about right.

 

"in an old epoxy"
Do you mean

1) the epoxy is already old at 13 years?

2) or do you think the epoxy might have been out of date, which is true, the epoxy used was Navy surplus from Plymouth, UK but was at the most not more than 5 years out of date. Temperatures don't vary much in a Plymouth warehouse throughout the year.

 

the oldest old epoxy story I have is someone using 28 year old epoxy. Originally the epoxy was used by a father and son to build a boat. 28 years later dad dies and son cleans out the garage and finds the epoxy. Decides to test....

that said, very often the resin (part A) will crystallize. heating it will melt the crystals and the epoxy resin is 'good to go'

paul
progressive epoxy polymers inc

 

I think this is probably a film thickness issue, more so than old epoxy.