epoxy/polyurethane uv resistance?

[ben570]

I'm looking at filling some cracks and voids in an ironbark slab with epoxy resin and then oiling/polyurethane it. Just as a bench seat but it is a nice piece of ironbark, has come up well sanding.

Question I have is. Are there any epoxy resins or polyurethane coats that will stand up to occasional exposure to the sun? For a few hours each day?

Also is it best to always apply a sanding sealer before a poly or epoxy coat?

Cheers

[BobL]

it depends how long (in years) you expect it to last without yellowing, and how much "yellowing" you call yellowing.

Under direct UV these finishes start to yellow quite quickly but because the colour develops slowly you don't notice it unless you hold a freshly finished surface up against the older one.

In terms of exposure, full sun for 6 hours a day will last less than 3 hours a day which will last for less than one hour a day, but even indirect (reflected) UV will eventually yellow these finishes.

Its very hard to predict but I can give you some examples.
A two pack epoxy finished bench in our back yard under dense tree cover but with direct early morning sun for about 3 hours a day (indirect sun for the rest) yellowed after about 9 months.
An outdoor table under a covered area exposed to about an hour of direct early morning summer sunlight and nothing in autumn/winter/spring went for abut 2 years before I noticed it had started to yellow.
Inside a house exposed to direct sun a couple of hours a day through closed windows, a PU finished occasional table eventually yellowed but it took around 10 years before I noticed it.

[dai sensei]

Question I have is. Are there any epoxy resins or polyurethane coats that will stand up to occasional exposure to the sun? For a few hours each day?

In addition to what Bob has said, do not believe manufacturers when they talk about UV inhibitors, they help slow it down but do not stop the yellowing.

Also is it best to always apply a sanding sealer before a poly or epoxy coat?

IMO yes. If you don't then trapped air within the timber continues to rise through your finish until it is dry. Using hot air blowers etc gets rid of those bubbles but there is always a few of the little bludgers you don't see that get trapped in the final finish that will annoy you for the rest of your life (talking from experience LOL)

[BobL]

The rate of yellowing also depends where you are in Oz. In southern Tassie it won't yellow as fast as in Darwin. I see you are inland NSW where the UV index will be on the high side.