Gelcoat on top of epoxy?

[bob26]

Can I use flowcoat/polyester gelcoat resin to finish off epoxy laminate repair (reattaching ply bulkheads which had rotted and filling defunct transducer holes under the floor)? I used epoxy to saturate the wood and for its extra stickiness.


If so, do I need to leave epoxy to cure fully first? Over polyester resin laminate I'd be inclined to add while laminate still a bit green so as to enable it to chemically bond.

If they are not chemically compatible presumably I let the epoxy go off first?

<hr width=100% size=1>

 

[oldsaltoz]

G'day Bob26,

Polyester does not stick real well to epoxy, even epoxy does not stick real well to epoxy if it has cured and not been washed before the next layer is applied, it leaves a wax deposit when curing, easy to remove, just run freshwater over it and rub with a plastic kitchen scourer, when the water stops forming beads it's clean, only takes a few minutes.

Epoxy based gel coat is readily available and will stick, perhaps you can use the polyester for minor repairs on your hull and deck later.

Most coatings can be applied to epoxy when still tacky, even antifoul to provide a good bond, but not so many will take to polyester.

Good luck, and I hope this helps.

<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[snowleopard]

epoxy is an adhesive so will stick to most things including polyester resin. Polyester is not an adhesive so does not bond well except to other polyester.

in other words you can put wet epoxy onto cured polyester but not the other way round. if you want to coat on top of epoxy, use more epoxy, with a suitable filler powder if you need to thicken it.

all of the above require that you get rid of the surface wax. i have experimented with washing (water and acetone) on polyester resin and have found that i can't get a bond without abrading. i haven't tried washing epoxy but the experts i have worked with always grind the surface before applying fresh resin.

<hr width=100% size=1>

 

[stuartw]

I'm sorry I have to dissagree with both of you.
Polyester, or in fact any resin will stick to most things, apart from the polyolefin plastics, like nylon, polythene etc.

The real problem for resins, going onto a smooth surface without some form of surface treatment, is their inabilty to key into it. The most effective way to bond one resin to another, and it does not matter which way round it is, is to provide a mechanical key. This can take the form of shot blasting (impracticable in this case) or the easiest thing to do in situ, is some form of heavy agrading. Basically you've got to make a mess of the surface, such that there are deep striations in as random a way as possible. Polyester resin sticks to glass fibre matting or rovings because in has a huge surface area to cling onto. Put polyester onto a glass sheet and it will peel off, unless it has been treated with a glass primer.
The reason that epoxy has this waxy appearance, is not that it contains a surface waxy deposit, is that it flows out and cures into a glassy surface with very few surface irregularities, and with low surface energy. A property uitilised to reduce hydrodynamic friction in certain types of pumps.

To summarise: The epoxy must be fully cured- at least 24hrs, then abraded, cleaned off with acetone, allowed to dry, then the polyester gelcoat worked into the surface, to wet it out. Further coats can then be applied to improve the finish.
Incidentaly, polyester resins are also used as adhesives, as well as epoxies being used in paints etc.


<hr width=100% size=1>

 

[oldsaltoz]

Washing off the residue after curing should be done prior to sanding or grinding cured resins, this is to reduce the risk of spreading the wax and thus contaminating the area to be re-coated.

Over 50% of delaminating problems can be traced back to re-coating levels, so take care when doing your preparation for the next lay-up.

Avagoodweekend...

<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Paul_H]

Agreed about the keying epoxy surface to get subsequent stuff to stick. Chemical bonding a new layer of epoxy has to be done within minutes of previous layer starting to go off. There is potentially a problem putting poly resin on epoxy. I once did a repair using Blue Gee epoxy which was allowed to harden and then finished with poly gel coat. After two days the gel coat was still like chewing gum but the remains in the mix pot had gone off within an hour. I was informed by the chap at Blue Gee its due to a particular chemical (cant remember what) in the epoxy leaching out and preventing the poly from going off. It can take weeks for the chemical to disperse and washing with acetone doesnt help either. Had to dig out the repair and do it all with poly resin. Dont know if this would apply to other epoxy resins.

<hr width=100% size=1>