How do you FORCE FCPx to re-render a clip?

Question

here's the problem.
Final Cut Pro X FREQUENTLY screws up transition renders. Did I say FREQUENTLY? It CANNOT be stressed enough, that FCPx DOES SOMETHING BAD to its transition renders Quite Often.
ok, until Apple fixes that nightmare, I want a workaround. I thought I had it:
if you hit control+r you tell FCPx to 'render selection.'  its right there in the menu.
but its a lie. what it SHOULD read in the menu is: 'render selection, if it is not already marked as having been rendered.'  This is a problem for me, because FCPx FREQUENTLY SCREWS UP TRANSITION RENDERS*.
*ok, I think I've said it enough, time for an example:
imagine a crossfade, standard, stock crossfade, using the standard timing... EVERYTHING is standard. Well, How would you feel if you were scrubbing an otherwise perfect edit... to find that your crossfades... ALL OF THEM... pop the resulting image, into the first frame of the transition? I bet you'd be mad.
How would you feel if you HAD NO RELIABLE WAY TO FORCE FCPx TO RE-RENDER THOSE TRANSITIONS?  You'd be livid. Lets put some more firewood on here... How would you feel if this happened randomly, all the time?  I really do mean ALL THE TIME. I can scarcely look at FCPx without it screwing up the transitions of my edit.
point is. FCPx has a wonderful editor. its a joy to use. But its chock FULL of bugs, and its so SMUG that it doesn't provide ANY workarounds. The Documentation is a joke. So I'm here... trying to find a way To FORCE FCPx to re-render something IT thinks is already rendered. the smug jerk.

Answer 1

OP here.
I'd like to make a few comments.
Apple has been slowly fixing critical bugs in FCPx, I'm happy with the direction things are headed. But we still have a long way to go.
the "delete Project Render Files" option is a late edition, and I "think" it is meant to address this specific issue. I find however that in practice, it introduces as many problems as it solves. deleting the project render files forces a re-render on the entire timeline, and as stated way back in the OP, FCPX screws up transition renders. 
Let me dig into that a little bit. Randomly, FCPx will cut out the first frame of a transiton, creating a jarring full frame jump (as it skips to the second frame) just at the front of a transition, and fill the last frame with black. All I want to do at this maddening point is force FCPX to re-render specific parts of the show, one at a time, because if I ask it to render the other stuff, invariably I will be subjected to the same amount of render bugs I am trying to eliminate. (or more) FCPX does this randomly, so if I can force it to re-render only a specific segment, there's a chance it won't render that bit incorrectly. And with finite control, I can step through and fix each transition, forcing as many re-renders as necessary, always getting closer to done, rather than starting from scratch at every step.
so, the tip is appreciated, it just doesn't answer my needs. I like FCPX, and sometimes its a dream but it is still riddled with critical bugs and the lack of percision control over the system is a design flaw, IMHO.