Filling gaps between timber floor boards can lead to disaster and is best avoided. Sanded and polished timber floors tend to swell slightly during the humid months of the year and shrink again in the drier months. This creates movement in the timber floor. If you fill the floor when the gaps are at their widest and then encounter some humid weather, the floors will swell, closing the gap and forcing the filling out of the grooves and cracking the timber floor finish. This looks far worse than gaps in the polished timber floors. If you can't live with the gaps, laying a engineered timber floor over the top may be your best choice. Painting your floors white is also best to avoid as this only accentuates the size of the gaps. Good luck with your decision. Engineered timber floors

i added a dining room extension 2 years ago and chose cyprus pine flooring to match the existing timber floor of my old cottage. Im near beaches so decided to lime wash the floor. 2 years down the track whilst i really love the look i can tell you the finish does not last. It is not even a high traffic area and you have to keep an eye on it to keep it clean of any little marks so they dont damage the surface of the wash. So my advice would be whilst a really beautiful look avoid because it does not go the distance.

Consider a burnished oil finish. I believe this to be superior to layers of polyurethane which often cause boards to stick together and show the scratches. It allows boards to expand and contract. If scratched just use oil over thecsctratch and it will disappear. This would still be done by a professional.

i was once told that "any wood matches any other wood" and that's helped me live happily with mismatched timbers for many years..of course it's probably possible to sand and stain the floor (ask a laminate timber floor supplier) but it'll be a tedious waste of sweat and tears for an uncertain result..what about make sure the walls, ceiling, architraves and skirting boards all look great and don't worry about under the hearth if it's not being moved, maybe add a matching trim around the hearth and plan a new floor for "later"