I wouldn't, no.

If you're that proud of your 220 grit glassed interior, simply mask off the area that would be in danger of scuffing when you go to sand (80 grit recommened) where fillets will be applied then get on with proper stitch & glue construction techniques.

If the designer had felt 5200 was adequate for filleting structural components into place I'd think they'd have had good reason to include that in the assembly manual. 

Don't interpret this to mean I don't think highly of 5200 (or Sikaflex for that matter) 'cause I do. They're both fine products when used as directed but neither is a proper substitute for structural epoxy.

Besides that, your cockpit's gonna need varnish or paint finish if that epoxy's gonna stand up to UV exposure in use. Neither 5200 nor Sikaflex welcome having such finishing materials applied on top.

Paint the fillets while they're soft with a chip brush dipped in unthickened epoxy. They'll end up glass smooth, ready for a 220 grit sanding to make the varnish stick.

Laszlo

   I wouldn't.  I tried the stuff once on a few fillets on a canoe i was building, and after a couple of seasons, the fillets started turning black and moldy.  

Interesting post kurlanny, thanks. I'd bought some 5200 for bedding hardware on my Waterlust build, never got around to using it. I've used SikaFlex before, haven't yet noted anything like what you describe with that product but it's in my basement, used to fillet around the basement floor joint where it meets the poured-concrete wall.

I'd been getting moisture wicking up through that joint years ago, thought perhaps 'caulking' the seam (along with several other things that needed doing down there) would help eliminate that problem when it rained. I used SikaFlex for this, worked great and indeed I haven't seen but one minor leak since then. 

   Tried painting straight epoxy on my soft fillets and they turned out better than expected which is rare. Thanks Laszlo! my fingers are especially grateful for that great tip. 

SB
 

  Spclark........" I'd bought some 5200 for bedding hardware ..."  Not to say you'd never get it off, but it is more difficult.  Use 4200, it  works better for hardware bedding. It stays softer.  

San Bernard.... I would NOT use 5200 in place of epoxy.  The 5200 really sticks, and is difficult to remove, but in my opinon and experience it isn't structural.   Given a joint with too much space it could flex in time and tear. 

   HOWEVER ........ That is exactly what we used to put the foam bulkheads in the stripper Petrel Play we finished last year. Those foam bulkheads were 2 to 3 inches thick and a force fit. A much different thing than a plywood bulkhead.  

Thanks Grumps, I think the removal reputation 5200 has is why I never cracked those tubes. I'll look for the 4200 stuff instead once I have need of some. Been using butyl roofing 'tape' instead, seems to work well enough and easy to find.

And I agree 100% on using sealant for foam 'bulkheads' which aren't really structural as are those made from plywood. Use what works best, most appropriate for the task.