I'm using Path->Trace bitmap to vectorize a "complex" image (meaning no geometric figure), the result is exactly what I want but the background it's transparent.
I'd like to fill it with white.
Fill doesn't work because it leaves a transparent line, and making the stroke bigger won't work since my figure it's curved and it either goes out of its outline or leave a transparent line.
It has to be placed on an image with a different color than white so the white must be within its limits.
I've tried to use a white rectangle and the path->division or the others, but i still can't find the right way to do it, it may be a combination of them
I've attached a sample of the traced bitmap

If I understand your requirements correctly, here's what I would do:

1) Duplicate the path (Ctrl-D)
2) Path > Break Apart (Ctrl-Shift-K)
3) Path > Union (Ctrl-+ (on the numeric keypad))
4) Set the fill to white
5) Path > Inset to pull it in from the edges a little. If that pulls it in too much, undo and use Path > Dynamic Offset instead so that you've got more control over the amount it moves by.
6) Send to back to put it behind the original trace.

Thanks it's almost perfect! Dont even need to change the inset, just a little area which was not enclosed now it's still transaparent, but now I can use more "raw" methods, I'l just try to change the brightness cutoff or cut a piece, many thanks again