caulk
What do I need to do in order to caulk this huge gap above an arched window?
Should I put some sort of barrier on top of the wood window frame and pipe cement to fill the gap. It is too large for even the biggest backer rod I can find. I am using Quad caulk and various sizes and lengths of backer rod on the other windows as their gaps are much smaller.
While I'm tempted to say the difference is just marketing hype, you can use either one, there is a very slight difference in formulation. The window and door stuff has slightly less petroleum distillates in the formula, according to MSDS information at nih.gov. Petroleum distillates will evaporate during curing, so I'm unsure why there's a difference. The difference is so small however, I'm confident you can use one or the other with no noticeable difference in the final installation.
I was going to say just acrylic latex (aka 'alex'). But since you don't like that or silicone, how about alex + silicone? :) For example:
The product is intended to address the relative weaknesses of each component. It's paintable, but supposed to be more flexible and better able to expand and contract with temp change.
Best Answer
I'd be tempted to try and get a piece of wood in there! Cut a piece over length and leave it out in the rain leaning against a wall etc. Wet wood will eventually bend (or you can encourage it over a couple of blocks by adding a bag of sand etc.) and get something the approximate curvature you're looking for and then cut it to length.
Not the quickest solution, but cement will crack up eventually as the window moves about through the seasons.
If it's too thick to bend naturally, (it looks to be on the upper limit at around 3/4 inch or so) you could laminate a piece up. Run some timber through a table saw to get 2 or 3mm thick strips and then glue them up over a former the same radius as the window. Again, not quick but oh, so pretty!