Finishing window "jam" on pole building

[mod600]

I'm still in the middle of finishing my pole building shop. 32 x 64 x 10 with divider wall to cut in half, so 32x32 cold storage and a 32x32 finished shop side. Divider wall will have 9' wide over-head door to roll toys from side to side. Anyways, back to my question. I have 3 windows in the shop side, I have 2x4 purlins on the inside of the poles for my OSB sheeting. I'm planning on putting J-Trim around the doors and windows, just like they did on the outside. I'm planning to use "L-6 Fascia" trim to line the window jams and then use the J-trim on the wall to cover everything up. However, I'm not sure how to finish it between the L-6 Fascia and where the window frame is. There is a little gap between the roughed in window opening and where the window frame is. They used some thin foam between the opening and window frame that the window actually sits on. On my buddies pole shed, they did the same thing, and then used some composite looking white trim (2"x1/4"?) that went on top of the L6 trim and butted up against the window frame. I was thinking of doing this, looked like a clean way to finish it off. But what is that trim called? I looked online on Menards & Home Depot and can't find it. One builder suggested just caulking on top of the L-6 trim where it meets the window frame. Not sure how well that'd look/work. So if anyone knows what that trim is called or if you have any other ideas of how to finished the area between the window frame L-6 trim...please share.
Thanks!
mod600

 

[RickP]

Our house is trimmed with white pvc board. It's available in various thicknesses and widths up to a 4x8 sheet. You can cut it/rip it with a regular saw, but it makes a ****** mess (white shavings everywhere).

Here's a pre-made trim board that's a little wider than you need:
Home Depot

 

[paulrey27]

Just for clarification... the jamB you're referring to that need trimmed or whatever is noted as trim in the image below?

 

[mod600]

RickP - Yea that looks kinda like what I want. HD has 1/4 x 1-3/4 lattice that is PVC, and looks like it might be what I want.

paulrey278 - No, by jam, I mean the returns & sill area in that image. Basically how to cover up the roughed in window opening.

 

[RickP]

RickP - Yea that looks kinda like what I want. HD has 1/4 x 1-3/4 lattice that is PVC, and looks like it might be what I want.

Good deal. I really like using PVC trim outside wherever I'm worried about water damage. And if you use it inside your shop, painting it would be optional. So you'd have even less maintenance than I do!

 

[IHI]

Get some PVC 1/4 round and make is snazzy.
Buy some composite or PVC 1x6x3/4" and rip strips to push up against window and nail off into jamb.

Get a break or have somebody come over and bend some aluminum/steel in the color you want in a Z shape so it goes up the window to hide the gap, covers the jamb, then bends down the wall to cover the rough edge of the steel siding that will **** upto the jamb on all 4 sides for a all in one look.

Quite a few different ways to do it and make it look nice, just all what you want to do.