Polyurethane over epoxy problem

[Lance Reid]

Hi all, I am currently finishing off a coffee table that has been epoxy resined and am putting a coating of polyurethane on.
Most of the surface comes up good but there are a few places where the urethane either leaves a dimple, as if the urethane is being rejected by the resin, or the poly goes all wrinkly.
Is this likely to be a problem with the resin? I've ground down into the resin to try and get a key for the poly, but still have a problem.
Thanks for any ideas.DSCF2499.JPGDSCF2498 (640x480).jpg

[Big Shed]

Some more info needed.

How old is the epoxy, is it reasonably fresh or has the table been used as furniture before you decided to put the "polyurethane" on it. If so, it has probably been sprayed with something like Mr Sheen which has silicone in it and you are getting fish eyes because of this.

I am assuming you are using a polurethane single pack varnish not a true 2 pack polyurethane?

[Lance Reid]

The epoxy is only about a year old and hasn't had anything else put over it, and the poly is a single pot by Cabot.

[Big Shed]

OK, so how old is the epoxy. Sounds like it may not have cured completely in places, perhaps due to insufficient mixing of the 2 components.

[Lance Reid]

Not too sure exactly how old the resin components were, but it has only been on the table a year.
Might have to grind out a bit deeper and build it up again.

[hurcorh]

Please excuse my ignorance, but if the entire top is epoxy covered, how come you want to put poly over the epoxy? I assume if the entire surface is finished with epoxy that this was the intended top finish for the timber. I understand putting epoxy into cracks / voids and then finishing over that but to put poly over a top that is fully coated in epoxy seems to not have any purpose to me.

My guess is, poly isn't supposed to bond to epoxy. It's made for timber so with that epoxy barrier there, it is causing you troubles.
Cheers.

[Nanigai]

Hurcorh it is very common to cover epoxy with poly for UV protection, especially on boats as it is not UV stable.
Don't know if that's the reason for it's use here though but there is obviously a problem somewhere and its probably the epoxy as mentioned above.
Cheers, Ian

[hurcorh]

Thanks Ian. I wasn't sure on that one.

[Lance Reid]

Had epoxied to level off the surface.

[Enfield Guy]

Just a shot in the dark, but, maybe the underlying surface is not as flat as you think? It certainly doesn't look like fish eye to me. It looks to me like a full gloss finish showing up faults in surface prep. Have you put a straight edge across the top? Is it really flat, or are there low points where the apparent defects are?

Just sayin'

[Lance Reid]

Previously had had another coat on and the poly was level, came back about an hour later and there were fish eyes, I think that's what you mean when the poly moves away from an area and leaves a dimple?

[fdpeterg]

If your timber is harder than the epoxy filler and you have used a soft sanding pad on your sander, particularly if you tended to press hard when sanding, you can cause an indentation wherever the epoxy forms the surface. When wet, the poly will self level, but on drying, it will contract and show an indentation.

The solution is to sand with a hard backing to your sander (e.g. Festool provide such an option) that will not "scoop" out the softer epoxy. Otherwise, a hard, dead flat hand sanding block is the way to go. This also preserves the table edges from excessive, unintended rounding over.


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