Polyester gelcoat

Hi Guys,

Im sure this topic has been discussed however i cant see the search bar :!:

How good is the adhesion of polyester gel coat and epoxy resin? What are the common problems around this.

Some people say it is important to use a vinyl ester tie coat between the polyester and epoxy? However is this necessary?

Thanks
Walla

 

Hi Walla,

Here are some previous threads discussing polyester gelcoat over epoxy, as a starting point:

• http://www.boatdesign.net/forums/fi...t-building/gel-coat-vs-epoxy-resin-50596.html
• http://www.boatdesign.net/forums/fi...system-epoxy-incompatible-gel-coat-27544.html
• http://www.boatdesign.net/forums/fi...-building/will-gelcoat-stick-epoxy-38685.html
• http://www.boatdesign.net/forums/boat-building/kevlar-epoxy-canoe-gelcoat-46985.html
• http://www.boatdesign.net/forums/fi...building/epoxy-polyester-conundrum-44309.html
• http://www.boatdesign.net/forums/boat-design/gelcoat-over-epoxy-2073.html

 

Lots of good reading there.


It's one of those things that the reply is, "it depends".

There are many different epoxies and there are many different polyester gel coats, some can be more compatible than others, and it can also vary depending on the techniques used to achieve the bond. Some report good results, some don't. Tie coats may help, but I don't trust them that much, and I sell them.

 

Thanks to both of you for that info,

Ondarvr you stated in one of the posts you had problems laying the epoxy onto a mould which had gelcoat on? I know gelcoat is terrible to stick to epoxy but not so much the other way around.

I was just wondering how the bond is usually from an epoxy onto gelcoat in a mould. How important is a tie coat layer of vinyl ester inbetween the epoxy and the gelcoat? Is it actually necessary if the mechanical bond of epoxies are so good? Or does it bond through a chemical bond

Im really over the mission of fairing and spraying and i want to avoid it by using gelcoat however i would be using a prime 20 LV epoxy so I think I should read their spec sheets and try to find out more info from my supplier.

Some people say its necessary some dont, who knows

Anyways thanks for those topics Mod I thoroughly enjoyed reading them and the others.

Blessing
Walla

 

On the backside of the gel coat (any polyester) there will be layer of uncured material due air inhibition, this uncured layer is a barrier for an epoxy to get a good bond. Some gel coats are designed for epoxy to bond fairly well, but it's not easy to buy for a guy off the street and it's pricy.

This uncured layer is an aide to the bond when it's polyester to polyester.

I know people that have gel coated the mold, let it cure, then sand it by hand with 80 grit, this works, but is time consuming and there is a possibility of the gel coat releasing from the mold.

Duratec VE sanding primer would be a good option to use in a mold for an epoxy laminate, it may not be perfect, but it surface cures well and people frequently use it with epoxy. Epoxy seems to bond to it faily well, and it bonds to epoxy fairly well. The only issue is that it's just a primer type product and not a finish coat.