I use my refractometer for final gravity. I just use the spreadsheet to correct it. I was under the assumption most refractometers had either just the Brix scale, or both Brix and specific gravity scales on them. Mine has both. If your's only has SG, then there are conversion tools online...
Your refractometer is fine. All of them need a correction for final gravity. They are best used for OG, but with the correction applied they are ok. I think MoreBeer also has a spreadsheet that will correct the FG for you. It depends on how accurate you need to be. I like using a...
You need to correct the final gravity readings when using a refractometer. Your final gravity readings actually are going to be ok, after you correct them.
Are you using a hydrometer, or a refractometer to check your final gravity?
If you are using a refractometer, then you need to correct the final gravity reading.
It would be just like it. I know from experience. I tried adding sugar to some under carbed bottles once, and they just foamed out before I could cap them. Those carb tabs that you can add up to 5 per bottle might work though, if you put 2 or 3 in.