Decks, Patios, Porches and Docks - plywood ends

I am boxing in the wrought iron porch posts that "modernized" my 1915 bungalow in the 70's.
the eaves extend about 3 feet past the posts, so I am not too worried about heavy water issues.
I plan on using plywood, and priming and painting. My question is ---is there a way to treat the edges of the plywood so that the layers don't show up? or do I have to buy lots of trim to cover things up? (the cornerbead wood products are Very spendy!
any other options?
thanks!

Try looking at the paintable poly trim (near the wood trim, but normally in its own racks). The 8' sticks would probably be less than $3 or so. It would work fine for your usage.

Cut the edges at 45°. It is trickier then trim but you did say: is there a way to treat the edges of the plywood so that the layers don't show up? or do I have to buy lots of trim to cover things up

thanks for your responses. I have thought about the 45degree cuts.. but I have 8 upper posts X 4 sides, and 4 lower posts - a lot of cuts!.....the nonwood paintable trim might be an option...
what about some kind of filler or putty or plastic wood? would that hold up at all?

45` would look best, otherwise if you don't cover up the edges with trim, prime them well [use exterior oil primer] sand when dry and either cover the plywood edge with a thin layer of caulking or exterior spackling. If you use spackling the spackling should also be oil primed. That's not the best solution but I've made more than one less than stellar carpentry job look nice :D

Hey, that's your job!!! Making us look good.

If the corner beads are too expensive, look at 1X trim, in whatever width fits the columns best. Install a 1X2 or 1X3 flush with edge of the panel that has the exposed ply and then installing one that is an inch wider to cover the both plys AND the edge of the first trim board installed. Caulk the joints, prime and paint.

you moderators and regular contributors rock!
thanks!