Spider cracks in gelcoat below waterline

[CHolgerson]

Hi Guys,

I just looked at a fully restored 20' Bertram yesterday and the boat was having the bottom redone. They blasted off the bottom paint to gelcoat and I noticed that the gel coat had spider cracks throughout the bottom. Is this a normal tendency for a 1968 Bertram? And if so is barrier coating the bottom the next step? I did not notice any major signs of blisters or fiberglass damage other than the spider cracks in the gelcoat. I am currently looking to add another Bertram to our fleet.
Best Regards,
Chris

[CaptPatrick]

That's the nature of any old gelcoat and even more so if it was applied too thick.

All thermal resins will continue to cure long after the initial hardening process is complete. As this takes place over the years, the resin shrinks and, without reinforcement, at some point will crack because the surface tension exceeds the strength of the cured resin.

Once the cracking occurs, water can, and will, be absorbed into the matrix of the fiberglass below it. This is not to be confused with the cause of blistering, which is usually caused by an excess of uncured resin molecules that attract water and swell up.

While just cleaning and coating the age cracked surface with an epoxy barrier coat will help, the best practice is to sand off the old gelcoat to the point that it is so thin that you can start to "read" through to the color of the underlaying fiberglass. It's the thickness of the old gelcoat that aggravates the cracking problem, and left thick, new cracks will develop, even under the barrier coat.

Taking the gelcoat off completely will create a new problem. The purpose of gelcoat is to create a totally bubble free surface over the fiberglass substrate which will be riddled with microscopic to not so microscopic bubbles of air trapped in the layup process.

By removing the gelcoat completely the surface will have pinholes that have to be filled before a good finish can be achieved.

[bob lico]

on the subject ; capt. patrick what if boat comes from factory with painted hull like jupiter center consoles . do they first gel-cote to minimize micro. bubbles and then prime and paint ???

[CaptPatrick]

on the subject ; capt. patrick what if boat comes from factory with painted hull like jupiter center consoles . do they first gel-cote to minimize micro. bubbles and then prime and paint ???

Generally so... But the gelcoat layer is applied much thinner than would be used for a hull that has gelcoat as the final finish. I do this with all of my parts since they ultimately will be painted or re-gelcoated after installation.

[Bob H.]

Chris, My 66 had the same issue on the entire hull and bottom, took it down to thin light green film showing through, rolled on several layers of epoxy and pressed it into any pinholes that showed up, faired the entire bottom then epoxy with barrier additive, about 12 coats total, faired again...that should last another 45 years...BH

[CHolgerson]

Thank you guys. I found a different 20' Moppie and the sale is pending a sea trial and survey. I will keep everyone updated. Should be a fun addition to Pops 31' Bahia Mar.